Depends on forum niche and layout, on my forum, often 0. I can have 20,000 page impressions and 0 clicks, which is why I don't really use google on my forum anymore
Forums are notorious for low CTR. My static content pages get 10x the CTR that my forum does on the same topics. Forums attract regulars....and they get ad blind very fast.
Forums are the worst target for ads, regular member wont click them and learn not to notice them. however, if you place them between the posts or under the main navigation, for example, ppl can click by mistake, which works good since forums are getting browsed a lot...