Was There a Specified Period Between Declaring War and Attacking Before World War II?

There was no specified warning period. And in the case of Japan vs. the United States, there was a "timing" problem.

The attack on Pearl Harbor was a "surprise" attack. So not even the Japanese Ambassador was warned in advance when the attack would take place. He received a telegraph message at 1:00 p.m. Washington time, and with some embarrassment, rushed to the office of Secretary of State, Cordell Hull to say that he had just been notified of a state of war between Japan and the U.S.

Whereupon, Cordell Hull indicated that he knew; he had just been informed of the attack on Pearl Harbor; that is to say, even before the Japanese Ambassador.

@congusbongus covers it from a legal perspective, but there is a way to avoid the letter of the law: make it look like you're being attacked.

Two successful surprise attacks which bypassed the letter of the law in WWII come to mind: the German invasion of Poland, and the Soviet invasion of Finland. In both cases the invader staged false attacks, then claimed they were invading in self defense. This gave them a pretext for invading a peaceful and weaker neighbor, allowed them to claim their surprise attack is legal, and claim the moral high ground to whomever is listening.

Neither fooled anyone, except maybe themselves, but it did muddle the legal and political waters enough to slow and confuse the response. It allowed their allies to parrot the bogus claims of self-defense. And it allowed the people of the invading governments to tell themselves they're not the bad guys.

The Germans conducted Operation Himmler, a series of fake Polish raids into German territory. SS and SD troops, wearing Polish uniforms, would attack and vandalize German border positions and towns. To add authenticity, they'd leave behind dead bodies in Polish uniforms, usually prisoners from concentration camps.

Hitler used these fake raids as pretext for his invasion of Poland in a speech on Sept 1st.

I can no longer find any willingness on the part of the Polish Government to conduct serious negotiations with us. These proposals for mediation have failed because in the meanwhile there, first of all, came as an answer the sudden Polish general mobilization, followed by more Polish atrocities. These were again repeated last night. Recently in one night there were as many as twenty-one frontier incidents: last night there were fourteen, of which three were quite serious. I have, therefore, resolved to speak to Poland in the same language that Poland for months past has used toward us...

This night for the first time Polish regular soldiers fired on our own territory. Since 5:45 a. m. we have been returning the fire... I will continue this struggle, no matter against whom, until the safety of the Reich and its rights are secured.

The Germans openly attacked before dawn on Sept 1st without declaring war. They bombed Wieluń and bombarded and stormed Westerplatte in Danzig followed at 0800 by a general invasion. No declaration of war was issued, and as far as I know, no declaration of war was ever issued.

The Soviets did the same thing to Finland, but with a twist.

After months of unsuccessful negotiations with the Finns to strengthen their defenses around Leningrad (which the Finns saw, probably correctly, as prelude to losing their independence as had happened to the other Baltic States), the Soviets decided to invade. But there was this pesky non-aggression pact with Finland guaranteeing each others borders and they'd need some pretext to break it.

Despite clear and obvious preparations for invasion, like hacking roads and railroads out of the forest that went nowhere but straight to the Finnish border, and the obvious absurdity of tiny Finland attacking the massive Soviet army, the Soviets went ahead with a deception plan that nobody believed but the Soviets. They chose two deceptions.

First is that the Finns fired first, same as the Germans said about Poland, but much less elaborate. The Soviets shelled their own border town of Mainila and declared the Finns did it. The Finns responded by saying they dutifully observed the Soviets shell their own city, and that they had no artillery in range of Mainila; anticipating exactly this sort of incident, and not wanting their precious artillery overrun in the initial invasion, they had pulled their few artillery pieces back from the border.

The Finns proposed a neutral party investigate the incident. The Soviets refused, broke off diplomatic negotiations, and renounced the non-aggression pact.

Second, the day after they invaded the Soviets set up the puppet Finnish Democratic Republic (neither Finnish, nor Democratic, nor a Republic) barely inside Finland and declared it the legitimate government of Finland. They dragged out Otto Kuusinen, an old Finnish communist, to head it up and give it some semblance of legitimacy. Its capital was the Finnish town of Terijoki. The Soviets could now claim they were invited to support their Communist brothers. In reality, "government" existed to rubber stamp anything the Soviets needed to be made legal, and to maybe draw any remaining Finnish Communists to their cause.

To give you an idea how laughable this was, Terijoki was so close to the Soviet border that it is now called Zelenogorsk and is part of St Petersburg. Kuusinen had fled Finland in 1918 after the Finnish Civil War and hadn't returned since.

Nobody believed it but the Soviets, and they continued to double-down on this political theater to the detriment of both sides. The increasingly desperate negotiation attempts by Finland were rejected because the Soviets would only negotiate to the "legitimate" government in Terijoki. Rather than accept the Finnish offers giving them all they had asked for, they continued to throw more men into the Finnish meat grinder rather than lose face by admitting it was all a sham; something everyone already knew.

Again, no formal declaration of war by either side was issued during the Winter War. Two years later, on June 25th, 1941, Finland declared war on the Soviet Union beginning the Continuation War.

I think the real trick Japan missed is this one: invade someone much weaker than yourself, make sure it's over and you're well entrenched before anyone can react, and do it one weak country at a time.

The Germans quickly annexed Austria, then invaded Czechoslovakia which quickly capitulated, and then The Battle of Poland was over in a month, a speed previously unheard of in warfare. All these invasions were over well before Allied troops could be organized and arrive in force.

The Soviets had swallowed up the Baltic States one by one in creeping occupations. When it came to Finland, everyone expected it to be over quickly, including the Finns. But it unexpectedly dragged out three months. Unfortunately for the Finns, while the Soviets were condemned, the Allies dithered constantly about their support, going so far as to turn it into a stealth invasion of Norway.

In contrast, the Japanese started the Pacific War by attacking the colonies of most of the major European powers at once guaranteeing a military response. While they had intended to be well entrenched by the time the US recovered from Pearl Harbor, they had no intention (nor capability) of finishing them off.

The legality of a declaration of war was outlined in the Hague Convention of 1907, treaty (III). Article 2 deals with the timing:

The existence of a state of war must be notified to the neutral Powers without delay, and shall not take effect in regard to them until after the receipt of a notification, which may, however, be given by telegraph. Neutral Powers, nevertheless, cannot rely on the absence of notification if it is clearly established that they were in fact aware of the existence of a state of war.

There is no period required; the treaty only requires that the formal declaration is received before hostilities commence. The problem with Pearl Harbor was that this declaration (which was not really a formal declaration at all) was not fully received by the time of the attack - the lengthy declaration was still being transcribed via telegraph.

Since 1945 this has been superceded by the United Nations Charter, where wars must be authorised by the Security Council - the Gulf War being a good example.