You can put it up whenever, but don't expect any money even worth mentioning before you have at least a few hundred visitors per day. In adult, you can expect a cent or possibly a few cents per unique visitor, from adsense you should have some idea of click-through stats. Say a quite high number, 5% would click your ad, from a site where you get 500 visitors per day. that is 25 clicks per day, at 1 cent a piece you earn around a quarter per day, or some $7-$8 per month.
In other words, not much worth doing it with much lesser traffic. Unless, of course, you have many, many small sites doing 100 or 200 visitors per day, then you can play the aggregated numbers and still make a few bucks per day.
Or get some popunders going on your site, and get paid up to $3 per 1000 pops, if you get onlu 100 visits per day, that's 3000 visits per month and at $3 per 1000 popunders, you still make $9 per month. Great deal, sign up and check it out here:
http://www.fpctraffic2.com/webm.cgi?Adraco