I find myself apply bleach for a really long time, like 15-20 minutes in total, how long after I've applied my bleach should I keep my hair under the showercap? 40 minutes seem to harsh... 25?
You need to start timing from the moment that you start applying. Do a strand test first to work out how long you need to leave it on in total. If it is taking you 20 minutes to do all of your hair consider just doing half at a time, quick application is the secret to getting an even result.