how much damage does window condensation cause
The critical point here is that your interior humidity level should be based on the outside temperature. At zero Fahrenheit you should be down around 25% inside, or maybe lower. 

That said, it's difficult to eliminate all window condensation while surviving as a human. Weather can change abruptly, and it's difficult to get your house to keep up. 

When things get very wet I'll go around and wipe up the liquid water each morning before it starts puddling. Wood damage occurs fairly easily. A bit of fog isn't a big deal.