I go about 6-7 weeks. I have an undercut and my hair is dyed, so that's about as long as I can go before the roots get bad and the undercut gets long enough to curl. I'm trying to grow the rest of my hair out but it seems like it always needs to be trimmed for split ends just about the same amount as it has grown. I really should bite the bullet and get better quality shampoo/conditioner, but I find the stuff that strengthens and smooths hair also makes me break out (maybe the added keratin?), so it's like I have to chose between having silky hair or (relatively) clear skin.