I've started some spinach from seed indoors and I'm not sure how many hours of light to provide now (at the seedling stage) and later (as the plants start to mature).

I have it under a dual T-5 fixture placed 4 to 6 inches away from the seedlings.

I know that spinach will bolt if the "day length" is too long, but when does this become a concern? Normally I give indoor vegetable seedlings 14 to 16 hours of light. I suspect this is fine for early spinach seedlings but I am not sure when to start cutting back on the hours to avoid premature bolting -- or how much to cut it down to once the spinach reaches that point. 8 hours? 12 hours?

Any suggestions based on knowledge or prior experience would be greatly appreciated. -- Eric