Rate of growth depends on many factors. Some of these factors are:
Quality of food
Quality of water (frequent waterchanges help because fish give off hormones that inhibit growth, so the waterchanges keep those from building up.)
Genetics
Two or three waterchanges a week and a nice variety of high protein and high veggie foods would go a long ways towards speeding up their growth.
1 month or so should see them large enough to go in with adults safely. 3-4 months should see them to nearly adult size, and 6-7 months should see them to full size.
