What damage can one expect if a dirty air filter.

You likely won't notice anything, just like if there was no air filter in it. Leaves and dust are going to go right through to the exhaust. Bugs are going to get roasted. Dirt will scratch the cylinder walls but even that won't show up right away. Over a long period of time the scratched cylinder walls will reduce compression slightly and lead to burning oil, just like on any high-mileage engine. I don't think I'd panic over the mishap but I would certainly try to not do it again.