[Online Media] Realistic Expectations for Ad Revenue, Given Traffic?

Hey all,

I work with a small-to-moderate-sized website. I haven't yet turned on advertising. I'm trying to get some realistic Ad Revenue scenarios.

I know this varies A LOT on a case-by-case basis, and it's impossible to give a one-size-fits-all answer.

That said, does anyone have any tips on how to find realistic projections? (As in, I tell you that I have X thousand unique visits a month, with 5 minutes average time on site, and you tell me that, in a ballpark estimate, it could translate to Y thousand dollars per month?)

Sort of a specific request, I know. Thanks in advance for any help. (If email is easier, I'm jeff@jeffwilser.com.)