I heard from a friend that if you want a darker shade when dying hair, you should leave the dye on for a longer period. However, I also am aware that this can damage the scalp, and increase risks of hair breakage.
But approximately, how long should I leave the dye on for? The box says 30-40 minutes, but is longer okay? (I have black dye by the way) Thanks.

Black will cover most colors in the time that is allotted on the box or leaflet in the box. Leave it on for ONLY the amount of time indicated on the packaging.

black hair dye will cover all hair color and you should leave it on for the time the box says especially if it's a chemical dye there are some hair dyes which you are supposed to leave on for as long as you like, like neon or very bright colors like manic panic or fudge paintbox brands I find they go pretty much used out in 6 hours but that's because their 100% herbal and actrually kind to your hair DO NOT leave chemicals on for that long it'll kill your hair and it may break or go bald in all respect your friends WRONG black dye doesn't go darker in shades it is an original color and stays that way. I hope I've been a lot of help =]

You should always refer to the Manufactures instructions. Failure to do so may cause depression, restlessness, shortness of breath and a realization that your life didn't turn out quite the way you wanted it to.