Depends what techniques you are using. Since I used a technique of LST, Supercropping and topping to get the plants as big as possible while still vegging, I let them veg for around 4 months before putting them into flowering (first 4 months 24 hour light, now 12/12).

This has given some rather fat bushy plants with tons of new stems / colas etc, with the delight of smoking material during these months due to trimming off fan leaves frequently to allow for better light reachings new nodes and forming new stems / colas and bud sites etc. The fan leaves themselves are very potent in my case so smoking them is great and due to the way I have done this they produce a LOT of leaves that can be trimmed off regularly without harming the plant or reducing any growth (it will be better for the plant to trim them off in either case or else it will be too much leaf growth covering the "good spots").