I once tried going for months without washing my hair when I was 19. It all came to a sad and sorry end when I was interrailing and a pigeon shat in my hair near the Trevi Fountain in Rome (who's that anyway?). The barber that had the unseemly task of shearing my matted and shitty locks from my head (Andrews in Covent Garden) refused to do anything but shave the lot off wearing rubber gloves and using an electric shaver attached to a six foot pole.
Well, this trip I haven't deliberately been trying not to wash my hair but since Mexico I just haven't needed to. Don't know what it is, maybe the salty water from surfing or swimming all the time, I'm really not sure. Well, that is until San Francisco. Somehow the change of country, temperature, scenery, local beer or whatever upset the natural but fragile equilibrium I had achieved and I wasn't going to wait until some passing bird took a pot shot at me to necessitate remedial action - I gave my hair a wash. So you'll be pleased to know I am clean headed and no longer have the growing odour of a dirty tramp or at least no more than normal.