It depends on how often they are used, and whether or not you allow the children to eat in the car. We use the carseats daily, and I do not let my children eat in the car (except during driving trips to Malaysia). So, I would say the car seats are washed around every 3 - 6months. The same goes for the pram cover. If you allow the children to eat in the car (think sticky fingers, food crumbs, etc)... it might be a good idea to wash them more often.
Car seats could be breeding ground for bacteria and germs. Also, the food crumbs would invite nasty pests like ants and crocroaches.
A note of advice - Pay attention when you remove the seat covers... This ensures easy installation afterward! Most of the time, the covers dries pretty fast (within a good sunny day)... so no worries there.