How long can you leave a bottle of already mixed hair dye out and have still be effective
no more than a day really, when you leave it it starts to turn brown, then if you try to use it, you wont get any colour, it doesnt do anything but stay brown,
and when you wash it off, nothing happens
so its not going to be effective for anymore than a day really after you mix it
try to mix just what you need and leave the rest for later
or mix what you need in a different container
It wont last long at all - when hair dye mixes all together the particles collide and need to stay warm. Once everything is open it cools down and dries out. I learned that in hairstyling class - also I tried doing my moms hair a day after I used to dye mine and it didn't work because the heat wasn't kept in.
I think I remember reading on the package (the pink box of the clairol kind anyways)
that it isn't very effective after an hour.