I live in the land of no humidity. I wash mine once a week (if I'm not regularly sweaty from the gym) and TBH I reckon it'd do 2 weeks (but that's kinda yuck). When I go south to Sydney I have to wash it much more often.

I thought I had dry hair (since it's curly), until I went dark and then realised it was the colour not my very normal hair. If I straighten it though I do find it gets yucker quicker.