How long will it take a baby molly to be of size for release in my tank?

my mollies had babies (I found only one and saved it) its been about 6 weeks and he's growing but barely it seems. I moved him into a breeding divider thing today so he could have the air flow/heat of my tank..is there any way to secure the floating area to a side or something? Is there any way he could get loose? also how long does it take for him to be big enough to realease