Peroxides or Developers don't 'spoil' or go bad but they can deteriorate meaning the Hydrogen Peroxide breaks down to free oxygen and water causing the product to weaken (go from 10 Vol. to 9 Vol. or less) or it can dehydrate or lose water and get more 'concentrated' (go from 20 Vol. to 22 vol. or more) However, all ...

What happens if you leave in developer for too long?

Your mix will be too dry, and you won't be able to saturate the hair properly with product and you may also get uneven and patchy color results. You will not be lifting out enough of the hair's natural melanin. This will mean you may end up with darker hair than you wished for.

How long should Developer stay on?

If you are using 20 volume developer with a hair color (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 color become inactive after 30 minutes, but your scalp may start to feel irritated if you leave it on any longer.