Thread on basically this same question here:
Here's how I would estimate for a fairly straightforward site:
* Your site map - how many pages on your site.
* How many ad units on each page and what type (300x250, 160x600...)
* How many visitors and average page views they'll generate.
* How many page views do you estimate for each of the pages.
* Total ad impressions then that generates for each size ad.
* Going CPM rates for each size ad in your industry. Check on competitors, do some research on ad networks you might use to fill inventory like advertising.com (et.al.) or if you're going to use AdSense.
* Do the math and you'll come out with a number. Do a "best case", "worst case", "expected case" scenario. Sell 40% of your inventory, 75% of your inventory, 90% of your inventory if you're getting advertisers directly.
* Note that other variables will affect these rates. They include whether an ad is above the fold or not, context of the pages, etc. But this will give you a ballpark for business plan.