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.

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.