Depends on a few things, i'm finding anyways.
Basically depends on variety and quantity of feed available and heat.
In my case, kept at a steady 30deg, given carrot lettuce dogfood and fish flakes 2-3 times a week.........its been about a 4-6 week cycle roughly (from pinheads to adult size).
I'd be looking to speed it up further but fish flakes aint exactly cheap. The more i give them of it the faster they grow.
The food doesn't completely get consumed mind you 2-3 times a week but i change it over none the less mainly to keep it reasonably fresh for them......and compost the used.