There's no specific time. The fact that your nursing very frequently, and most of his feedings are 15-20 mins and he's gaining well says he's getting plenty of hindmilk. Think of the short feeds as snacks or getting fluid. Foremilk is important too - it's like baby water.
Here's a good explanation about hindmilk:
http://www.kellymom.com/bf/supply/fo...-hindmilk.html
Here's a good explanation about hindmilk:
http://www.kellymom.com/bf/supply/fo...-hindmilk.html