How many weeks into grow do you wait till it OK switch over to Hps lights? I plan on doing T5, 250 Mh then for flowing 400 Hps .
I would eliminate the 250 MH. Grow your plants under the T5 for as long as you would like to veg, then move under the 400w HPS. You just need to make sure the T5 is big enough for the amount of plants you are growing... same with the 400w too. No need to switch between three different lights.
I think after about 2 weeks you'll be fine to switch to the metal halide light. Then veg for however long you'd like and turn it over to the high pressure sodium light.
switch to the hps when you want to flower or when they show sex. there are some things that go into the when you want to flower, height,age,strain and growing stile.