This will be different for everyone! We recommend leaving the bleach in the hair for 15-45 minutes. Because some people will experience their desired lift in less time, always check the hair every 5 minutes or so throughout the process. If your hair is very dark and/or coarse, you will most likely require a longer processing time than someone with lighter and/or finer hair. Bleach will become less active as it dries out, and will typically stop processing after about 45 minutes. Keeping hair fully saturate will help prevent the bleach from drying out too early.
Bleach should be washed out as soon as your desired level of lift is achieved, and should never be left in for longer than 45 minutes at the most to prevent damage to the hair, and should be checked frequently throughout the bleaching process. If at any point you start to feel pain or discomfort beyond the usual slight itching or tingling on your scalp, the bleach should be rinsed out promptly.