I run a blog and my rule is when I have 30 posts up. Is this good? Or should I do it when something else happens. I know I should search for this but my search term is to long apparently
I'd say put adsense up when you have a steady stream of daily traffic. I personally wouldn't put it up unless I had at least 50 uniques/day from organic traffic.
You don't want to wait to reach some particular post count to put adsense ads you can start displaying right from the day you have created a site if you have Adsense Publisher ID...