Firstly it is totally up to you, if you have 3 feet vertical room, dont veg past 20 inches. more room you say? flower at any size you wish, the plants AT LEAST double in size(usually more than double) during flower, mostly 2-3 sometimes 4 weeks should be more than enough vegging(depending on everything you do/give), 5-6 weeks veg you could end up with a 6 footer, I myself wait till 15 inches, (since I bend them over cause I have less than 2.5ft vertical space but again, its completely up to you and really depends on all the other variables. It takes soooo long to flower(7-10+ weeks), 3 weeks veg should give you plenty of plant to finish with.
Just go by size, not time; thats good advice...