How long you should leave benzoyl peroxide on your skin will depend on the treatment you’re using.
If you’re using a benzoyl peroxide gel, you may need to leave the treatment on so it can work but if you’re using a wash gel you’ll usually need to rinse it off after a minute or two.
The best thing you can do is check the information that comes with your treatment, as it will explain exactly how long you need to leave it to do its thing and whether you’ll need to wash the treatment off afterwards.