I applied as soon as I got my domain and an index page online. I had no visitors and virtually no content on, but they approved me hehe. Now I'm only really getting 30-40 uniques a day, but its a niche site, and its my first, and thats visitors! weeee I guess google aren't too fussed as long as it looks like your moving content onto there at a good pace.
It's in Google's financial interest to take you in. As long as your website doesn't go against their terms of services, you won't have any problems.
i think abouth 100 or something,under that you wont get the earnings you expect and might get demotivated and delete the ads.
The question should go for what is the content of your site, how unique and valuable is your content, how often can you update it. Unique visitors don't guarantee you high earnings in adsense. You can drive untargeted 10,000 visitors per day and earn $3 from it but one can have 300 targeted visitors and earn $15 a day.