If Your Hair Can Handle It
If your hair is already blond, using peroxide shouldn't hurt it and will turn it a much lighter shade in just 10 to 20 minutes. Doing a couple of test strands on your hair's lightest and darkest areas before dyeing will tell you how long to leave it on for best results.
If Your Hair Can't Handle It
Over-dyeing makes hair dry and brittle. If you've been dyeing your hair frequently, it is better to go to a salon than apply peroxide on your hair and risk having it burned off at its weakest point. Darker hair colors frequently turn red with peroxide instead of the bleach blond most people are looking for.
If your hair is healthy, do a test strand to see how long to leave peroxide on. If your hair is not healthy or very dark, consult a professional first before applying peroxide, to avoid undesirable results.