If my 26 old son tends to breast feed for over 1 hour and I know I have enough milk, will this cause my milk to be less for next feed or increase?

>15min/side= nurture. Baby will extract 80-95% of the milk you have for a feed in 10-15min/side, so > 15min/side becomes a nurturing session. If you are making plenty this is fine. To increase volume you would cut the feeding time shorter so you could get more feeds in/day.Extended feeds can slow the movement of milk forward from the after-breast.