Mabel drinks 2 pints of milk each day. How many days does it take her to drink 1 gallon of milk?

There are 8 pints in a gallon, so Mabel drinks 8/2 = <<8/2=4>>4 pints per gallon.

Therefore, it takes Mabel 4 days to drink 1 gallon of milk. Answer: \boxed{4}.