There's some debate on that. One of our landing guys was wearing chaps that were on their fourth or fifth season without being washed. They had so much accumulated crud on them that they were black.
He was limbing a log with a 660 and it rolled on him, pushing the saw into his left leg above the knee. The chaps did just exactly what they were supposed to and stopped the saw cold. No damage to his leg.
All the years of oil and dirt and sawdust and snoose dribbles didn't seem to affect the
inside of the chaps at all.
That being said, if a guy wants to trade his chaps off when they get dirty or wash them I don't see anything wrong with that. Chaps are cheap, even the expensive ones are cheap when you consider the alternatives.
One side note...if you wash chaps often enough the orange color fades to a lovely shade of muted pink. Be prepared for comments.