The numbers I've heard and seen are 150,000 to 250,00 visitors per month will translate into $100 a day - $3,000 a month (US dollars) in income.
My Adsense account currently generates about $100 every other month and judging from my traffic numbers I think those numbers are about correct.
A LOT of traffic for some. Not so much for others.
Certainly there are wide variances depending on the niche(s) that you are pursuing.