At a year to a year and a half I cut my Jackson's feeding back by half. I do it about the time they start getting picky and they aren't growing any longer. Some people go to feeding the same amount every other day but I just feed half as much daily. Your females likely won't get picky so definitely cut them back by the 18 month mark.
I don't have the exact same species so I can't give you an exact amount of feeders and sizes.
The best way to judge their weight is to watch the base of their tail you don't want to see bones or ligaments through the skin but it shouldn't look like a balloon animal either but some where comfortably in-between.