How long to leave hair before dying it again?

I've dyed my hair purple, I love it a lot- but when I bought it i didn't realise I would need two boxes and we ran out- half my hair is brown/blond and the rest is purple! Haha... I've got two boxes of the same dye ready to dye to the rest of it. Do I have to wait before dying it? I want to dye it ASAP :)