Mmmm! this is a bit of an old chestnut.
I think it is safe to say that the surface finish of aircraft,particularly fighters,was kept in as good a condition as operational circumstances allowed. Here is a list of dos and don'ts from a manual circulated to ground crew during the war. Of particular interest is #12
In 1942 maintenence of the new smooth finishes was causing concern to the Air Ministry and manufacturers as evidenced in the final paragraph here. It was thought that it could be done better.
Both these documents are courtesy of the hard work and research of Edgar Brooks.
Aircraft undoubtedly became tired and worn but a dirty aircraft would be cleaned in pretty short order. You can find evidence of aircraft which are quite mucky particularly during periods of high work loads so it's really up to you.Remember that all service aircraft spent far more time on the ground,where they could be cleaned and serviced,than they did in the air.
Cheers
Steve
Cheers
Steve