The plants will let you know. They will start to fade lighter or show you some deficiency. That's when you know the time release nutes ran out. I grow in 3 gallon pots and I think I went about 5 weeks when I tried out Fox Farm soil. So I would imagine you are good for a few more.
How long you veg depends on a couple of different factors, one of the biggest factors is any height limits you may have in your flowering room, that will directly correspond with when you want to start flowering. I've noticed it to be beneficial indoors to flower in between 12 and 36 inches in most situations. So sounds like you're right on target with 3 feet as long as you have enough headroom and light.