If my creature is over 18...

[soul41]

Using the extreme growth map, how long would it take to get from -79% to the max.

 

[Kays]

-79% is about half grown so it might take some time. Since I've never seen the amount by which the growth rate is reduced it'll be hard to estimate exactly how long.

I say just let him grow naturally. I don't know he would turn out mentally if he's to spend that much time sleeping in his pen. Especially since he's a nature creature.