I dont think there is a limit. Or a minimum. When I applied for Adsense, I had just started my blog. I did it even before I started to write on any website, because I didnt want to lose out on money. I think it all has to do with Traffic, and how well people can navigate through your site.
If you want to get approved by Adsense Fast, I suggest you start a blog first, then move on to other websites. Its easier.
Hope this helps.