Most plants are ready to flower when they are 12 inches tall and/or about a month old. There isn't really a point when its been too long as long as your plant is healthy. Like the last reply said you definitly wanna make sure they recooperate and look healthy before you start flowering them. Some plants have been in the vegative stage for several years. 12 inches is the soonest you can flower them which means the sooner you'll have your shit...I would say that's the best time to start flowering. Of course everyone has a different way of doing shit. If you wanna let them get 2 ft tall before you flower then do it, but if they keep getting optimal nutrients, theres a good chance that plants gonna end up taller than you. Do you really wanna have to look up into your lights to trim and check the buds for ripening? I just started flowering a plant that had been veging for 3 months and doesn't look like its gonna yeild anymore than the ones that were a foot tall when i started flowering. I ended up cuting the bottom half of the branches off cause they weren't getting any light. Now it looks like a giant mushrooms with little buds sticking out of the cap. Its definitly not my best plant.