SEO theory suggests dropping ads until you have lots of traffic b/c with AdSense you provide random relevant exit points and they're not hooked on you yet. It's also distracting, takes away from the experience, etc.

In your case I think it's subtle enough not to detract.

Web page quality reduction caused by ads = (Some constant)

So, you plot that out and find out when the money derived exceeds the damage. Requires guesswork, of course.