How many minutes of daylight are gained per day in Mexico City?

That will vary in a sinusoidal pattern, but it depends on the date. From December 22 to December 23, you gain a few seconds of daylight. But it increases day by day, until on March 21 (the equinox) you're gaining 3 minutes per day. Then it starts to decrease, until from June 19 to June 20, it's only a few seconds more.

After the Summer Solstice around June 21, the amount of sunlight each day begins to decrease, in the same pattern.