it all depends on how much weed you want to end up with, the bigger your plant is when you start flowering, the more you'll end up with. Starting your plants flowering at less than 3 inches would be pointless. Wait until they are at least 10" tall, plants continue to put on height for about 1/3 of the flowering cycle. If you have the time and space, let em grow tall, crop 'em once or twice, and then go 12/12.

Peas out