Answers to Accumulated environmental damage to Hubble main mirror are inconclusive, as are answers to Do we have a photo of a micrometeorite impact on a lens?. However see the Tim Peake tweet of a meteor impact on a window of the ISS below, which for the purposes of this question won't qualify as full fledged optical surface, part of an optical system.
Per @Hobbes via now deleted post, that event is further discussed in Market Busness News' Crack on International Space Station window from flying debris.
Question: How fast do optical surfaces get dirty or damaged in space? Space is big and this might require an answer that distinguishes between very low Earth orbit (How low is VLEO? (FCC's newest approval for SpaceX)) and deep space, but I think it's best to have both in one place, rather than split this up into two question.
However if you would prefer to have this split because you'd like to address only one, let me know.
Source: Tim Peake tweet