about 6 weeks
I think it lasted about 6 weeks before it was significantly faded. I only had highlights so it wasn’t as obvious as if I had dyed all of my hair.
How to dye my hair blue
It made me
blue-haired 
"I didn't realize I would also be dyeing my bathroom blue"
How I did it: I'd heard a lot of horror stories about bleaching black hair, so I went to a hipster salon and had them do the bleaching and first dyeing. It took almost four hours for the foils and bleaching process, so I'm pretty sure I got my $100 worth. I chose a really dark color (Special Effects Blue Velvet) for a more subtle effect.
They don't make permanent blue dye, only semipermanent. I expected the color to gradually wash out, but I didn't realize how potent the washed out color would be. For the first two weeks (and of course every time after I redyed it) I'd have to bleach the tub after every shower to get the blue off. That got a bit tedious after a while. Mostly for that reason I didn't get it done again after the white ends grew out.
