2 1/2 months. IF you turn the carton over every day. That is; it is right side up one day, and upside down the next.
We did this in the tropical country I grew up in, in a rural area with no electricity. It works in temps up to 90F with 90% humidity. Don't know about higher temps than that.
If you just leave them right side up, they will still last a good 6 weeks.
If you did that with 10 degrees warmer and a little bit lower humidity, and turned them 4 or 5 times a day, for about 21 days, you'd end up with more chickens.