How long in veg stage, well, the answer is 'as long as you want'.
You should start to see alternating nodes within the next 2 weeks, meaning your plants are sexually mature and ready to flower.
Just FYI, if you are using a separate HPS for flower, not a switchable or conversion, you will want to run a much larger lamp- 1000 watts in flower is appropriate after 400w in veg. The ratio should be approximately 1:3 but it's not exact lol.