Many factors play into this but seedling growth is about 2-3 weeks and then veg can go in theory "forever". But somewhere between 2-6 weeks depending on strain, lights, your goal, grow space etc.
Plants are mature enough to flower when the plants starts to alternate the nodes. Some people go 12/12 from seed but the plant cannot flower until it becomes mature enough. It is totally up to you. Longer veg = bigger yield.