You can leave it on up to 50 minutes and then rinse out, shampoo and dry. We do suggest checking your hair after 30 minutes to see if you have achieved a high enough level of lift! If upon rinsing out the lightening kit you think you need to go even lighter, we suggest waiting 2-4 days and then lightening again.  We sure to monitor your hair closely as not to over process the hair. Since that would be your second time lightening, we suggest only leaving the lightening kit on for a max of 30 minutes and checking frequently!