Maximum productive time for a meditation sit?

well the legend says that the citta sits in samadhi for 7 days max, so strive for that.

for sati sampajanna, ie mindfulness, there is no need to sit. You do it when you eat, when you walk , when you clean the house, and it is the continuous activity: each hour of each day of each week you track thoughts and sanna and vedana, in order to swap bad thoughts, which are that

And what, bhikkhus, is the unwholesome? Destroying life, taking what is not given, misconduct in sensual pleasures, lying, malicious speech, frivolous talks, covetousness, ill-will and wrong view: this is called, bhikkhus, the unwholesome.

And what, bhikkhus, is the wholesome? Abstaing from destroying life, abstaining from taking what is not given, abstaining from misconduct in sensual pleasures, abstaining from lying, abstaining from malicious speech, abstaining from frivolous talks, non-covetousness, non-ill-will and right view: this is called, bhikkhus, the wholesome.

The fruits of mindfulness are that:

Bhikkhus, heedfulness should be practiced in four instances. Which four? Abandon bodily misconduct and develop bodily good conduct; do not be negligent towards it. Abandon verbal misconduct and develop verbal good conduct; do not be negligent towards it. Abandon mental misconduct and develop mental good conduct; do not be negligent towards it. Abandon wrong view and develop right view; do not be negligent towards it.

Bhikkhus, heedfulness, mindfulness and protection of the mind should be practiced by oneself in four instances. Which four? Heedfulness, mindfulness and protection of the mind should be practiced by oneself [thinking:] 'May my mind not become avid on account of things that induce avidity.' Heedfulness, mindfulness and protection of the mind should be practiced by oneself [thinking:] ' May my mind not become averse to things that induce aversion.' Heedfulness, mindfulness and protection of the mind should be practiced by oneself [thinking:] 'May my mind not become deluded on account of things that induce delusion.' Heedfulness, mindfulness and protection of the mind should be practiced by oneself [thinking:] 'May my mind not become intoxicated by things that intoxicate.'

When you get good at mindulfness, the citta will sit in samadhi and the sañña and vedana will become good, meritorious, and when you have those jhanas, you do vipassana like that

Venerable sir, is there a single quality declared by the Blessed One—the one who knows, the one who sees, worthy & rightly self-awakened—where the unreleased mind of a monk who dwells there heedful, ardent, & resolute becomes released, or his unended fermentations go to their total ending, or he attains the unexcelled security from the yoke that he had not attained before?”

“Yes, householder, there is…”

“And what is that one quality, venerable sir…?”

“There is the case, householder, where a monk, withdrawn from sensuality, withdrawn from unskillful qualities, enters & remains in the first jhana: rapture & pleasure born from withdrawal, accompanied by directed thought & evaluation. He reflects on this and discerns, ‘This first jhana is fabricated & intended. Now whatever is fabricated & intended is inconstant & subject to cessation.’ Staying right there, he reaches the ending of the mental fermentations. Or, if not, then—through this very Dhamma-passion, this Dhamma-delight, and from the total wasting away of the first five Fetters—he is due to be reborn [in the Pure Abodes], there to be totally unbound, never again to return from that world. https://suttacentral.net/an11.16/en/thanissaro

Sitting for an hour is the maximum productive time for a meditation sit. Sitting in a meditation position for longer than an hour can be unhealthy for the physical body and it can reduce the effectiveness of the meditation. I think in some traditions people are sitting for long periods of time but it requires special training. I think that in formal meditation walking must be done as much as sitting. So it is better to do walking meditation before and after the sitting meditation. Then when you return back to the sitting meditation, you'll be refreshed and you'll be able to meditate effectively.

The chances of enlightenment increases when the mindfulness becomes continous in daily life. So doing intensive meditation for many hours and keeping the mindfulness all day long is the gateway to liberation.

I find daily sits of an unbroken hour and a half to be optimum. The first half hour is normally just enough to settle my mind and loosen the hold that the day has on me. The next half hour is spent really honing in on my concentration object (i.e. cultivating sustained thought). It's here that my mind begins to unify into something resembling one-pointedness. The next half hour is where the magic happens, so to speak. During that part of the sit - and really depending on the day - is when my body starts to drop away and rapture begins to arise.

Some days I may push toward two to three hours, but unless you are in a retreat situation, those longer sits don't necessarily get much deeper. Your karmic obstacles simply have too much of a hold on your mind. Only extended periods of silence and multiple 8+ hour days of meditation will allow you to progress into jhana.

Side note: my teacher's teacher knew a monk who would just randomly sit for two days. 48 hours. No sleep, no food, no bathroom. In full lotus. I can't even imagine doing that!

At home I think sitting for 20 - 40 mins once or twice a day is ideal. If you want to sit for longer, then the best thing to do is to go on an organised retreat somewhere.

I would also suggest having a look at this series of articles by Shohaku Okumura (a dharma grandson of Kodo Sawaki), who says:

...zazen itself can be a poison and cause sickness. If our motivation to practise is influenced by the three poisons, that is, if we practise for the sake of making this person more important, more powerful, more enlightened or for anything else, then it is motivated by greed, ‘I want to get this or that.’ It may not be for wealth or power that we practise, but for something spiritual. If we practise in order to get something desirable, however, our zazen is generated by greed.

https://buddhismnow.com/2014/10/22/zazenshin-acupuncture-needle-of-zazen-shohaku-okumura/