Last year, we changed the ad frequency, and actually lowered it... but over time, it felt like that somehow meant *more* ads in the forums. When I received multiple page loads of ad-post-ad-post-ad, that was enough. So we've locked in ad frequency and no longer allow the ads to "fill" at a percentage rate. Now it's every 4-5 posts, no more than that, ever.
As always, if you have issues with ads, or want to voice a concern, please speak up.