Every other day. I would like to wash it daily but it clogs the drains - however, the natural oils are removed if washed daily and this can make the hair brittle and dry.
I get my hair cut once a month though I'd come back from the hairdressers with my head fairly itchy, even after a shampoo.
Perming hair should be done professionally as ordinary perming solutions have really dried out my hair in the past. I'd say this should be done every 3-4 months and a conditioner for permed hair used regularly.