If you are using 20 volume developer with a hair colour (as opposed to a lightener or bleaching agent) then 20 to 30 minutes is the ideal range to leave it on your hair. Not only does the colour become inactive after 30 minutes, but your scalp may start to feel irritated if you leave it on any longer.
Until You See Results
If you are using 20 volume developer to highlight your hair, the bleach will remain active with it until you rinse it out, therefore you can leave it on until you see your desired results. It is important to remember that bleach and developer can damage and dry the hair so the less time it's on, the better.
When it comes to using 20 volume developer on your hair, the manufacturer's instructions should be followed carefully to avoid damage to your hair and scalp. If you experience a burning sensation at any time during the process, immediately rinse off the product to avoid injury, even if it has only been on for a few minutes.