You can start flowering at anytime. Some people use a 12/12 light cycle from seed but the yields are lower. Yields are impossible to predict.

If your plants haven't reached maturity yet and you switch to a 12/12 cycle, it'll take a couple of weeks before you see pistils/preflowers with most strains. If you wait until you see alternating nodes and preflowers they will start blooming almost immediately after switching the lights.