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

1 gallon is equal to 8 pints.

Therefore, Ellen drinks 8 pints in 1 gallon of milk.
Since Ellen drinks 2 pints each day,
it will take her 8/2 = <<8/2=4>>4 days to drink 1 gallon of milk. Answer: \boxed{4}.