How many hours of light do plants need in veg and in flower?

Seedlings and cuttings do fine on 18 hours. Going to 24 hours doesn't help and just costs power; plants need a dark period. In flower, the 12 hours of darkness must be truly dark, since light leaks can disturb flowering. Pick a schedule and stick to it, because irregular cycles stress the plants.

A timer keeps it exact, and a tent with tight seams keeps the dark period dark.