Took me when I first started (website was already developed and established for two months till I added Adsense on it) about 1 week, not really sure... but this was a while ago and I trashed that site and re-built it on another name. Niche? I think it was entertainment forum? Not really to sure...
Truly speaking... i started in January 2006 with a blog, from 0.00 cents a day to slowly and steadily 10 cents a week, after i had my directory setup to serve these ads it was slowly 0.40 cents a day... and here in April 2007 nearly 15 months I managed to cross the 100$ I am happy and the payments are on hold
It took me about 3 months to begin with and now I'm making $300/month from that same site. It's all about traffic.
About 3 months, although dropped out of google and other engines so traffic has dropped meaning so has adsense.
haha why not take benjamins all day in other ways, adsense will cover teh cost of yoru domain and hosting unless you really know what you are doing and have high traffic or highly niche keyword sites.
It took me just over 3 months from a single blog. Considering the blog was in a niche I am not particularly knowledgeable in, I was quite pleased.
Well I started my blog on Jan. 2007 and first month I earned something $10 but than two months going wrost for me but April turn out biggest month for me and now I am closing $100.