How long did it take mail to get between Madrid and Mexico?

Columbus’s first voyage across the Atlantic to the New World in 1492 took more than two months. Crossing times did improve over time, but by the 1700s, sailing ships still needed six weeks or more to make the crossing.

Making an allowance for the overland transport of messages, it would be reasonable to estimate that mail would take at least 8 weeks / 2 months to travel from Madrid to Mexico.

However, we should also remember that through the age of sail, crossing times would also have been greatly effected by the weather, so an estimate of two months would still only be an average. As noted in this article,

An immigrant who made the journey in 1750 reported that it could take between eight and 12 weeks, while another who arrived in 1724 reported that the journey took six weeks and three days.

It looks like around 7-8 weeks ship time. See update below. (Inserted new info at top, ahead of the background info on the avisos ships and routes.)

--- UPDATE --- Finally found actual info on the travel time for the avisos. This book concerning the HMS Centurion actually lists the transit time for the avisos San Lorenzo as 41 days. This put the ship at Puerto Rico. Leaving there on Jan 31st, detained by British on Feb 3rd, and after rescue and various engagements reaches Santiago Cuba on Feb 12, after 12 more days. Then it apparently took two more weeks (inland Travel?) before the message was finally delivered to the Governor of Cuba, Juan de Prado,on Feb 26th 1762, informing him that Spain and Britain were at war. So 53 days travel time to Cuba, plus a couple more weeks delivery time. (And this just got us to Havana.)

Now, some background about the mail service.First of all, the correspondence between Spain and Mexico was carried by specialized vessels called avisos:

The communication between the Spanish government and the Viceroys in America was carried out by small and fast vessels called avisos that operated under the directions of the Secretariship of State. Initially, these ships were intended to announce in America and Spain the departure of the Flota, and the Galleons, respectively. However, they ended up carrying with them all the colonial correspondences. On 20 July 1718, the Crown established that an annual number of eight avisos would have to be employed to carry the colonial correspondence. Two years later, on 31 May 1720, the Consulado de Cadiz and the Crown signed an agreement.22 The Consulado agreed to add another eight vessels to work as avisos every year. By the 1730s, the avisos arrived at Buenos Aires, Cartagena de Indias and Havana on a monthly basis.

from Mobilizing resources for war: the British and Spanish intelligence systems...

Later, we see that the use of these packet-boats is reconfirmed:

Charles III in 1764 established monthly mail packet boats between Corunna and Havana and these were permitted to transport goods to the extent of half their cargo Every two months a similar packet boat went to Buenos Ayres and there were American post routes connected with it In 1765 came the great advance.

from: The Spanish Colonial System By Wilhelm Roscher

We can also learn a little more concerning the avisos because a couple of them have been found off the coast of Florida:

Ships thought to have wrecked off the Jupiter coast during the 1600s include the San Miguel Archangel and the San Francisco y San Antonio. Both ships were avisos, Spanish courier ships weighing 60 tons or less. The avisos were well-armed, but speed was their best defense, which allowed them to outrun larger, better-armed vessels. Avisos were primarily used to transport correspondence to and from the Spanish throne, but were known to carry goods and valuables.

from http://www.waymarking.com/waymarks/WMRP81_Jupiter_Inlet_Shipwrecks_1600_1700s

Now we can compare this size to that of the ships most often thought of when we think Spanish fleet, the Galleon

...while galleons were mostly under 500 tons, although the Manila galleons were to reach up to 2,000 tons.

I did find a little more describing the actual route generally followed by the trade fleet:

The ships departed from Seville (later from Cadiz) and sailed down the coast of Africa to the Canary Islands, where they stopped for supplies. They then turned west to take advantage of the trade winds and, after sailing about a month or more, entered the Caribbean southeast of Puerto Rico. Here the convoy split into two fleets: the Tierra Firme (Spanish name for the South American mainland) and the New Spain. The New Spain fleet sailed on to the port of Veracruz in New Spain (present day Mexico).

(and for the return voyage)

The two fleets met up in Havana and made preparations for the return trip to Spain. When they left Havana, the combined fleet sailed along the east coast of Florida and rode the Gulf Stream—a strong, warm ocean current that originates in the Gulf of Mexico and flows north along the east coast of the United States— north as far as Cape Canaveral before heading east towards Spain.

above from this National Park Service site