It depends very heavily on the web site and its keywords. For example, I have one web site (www.blazemp.com) that has a lot of related keywords and gets around 30,000 visitors per day from Google. That is because the keywords for that site are very popular, such as DVD burner (for example), and there are a lot of them. However, a site that I promote more than blazemp.com may only get 700 visitors per day from Google, even though it ranks just as well for its target keywords. That's because its keywords are not as popular and there are not as many of them that get a lot of traffic. In my opinion, as far as the amount of traffic a site gets from Google, it depends a lot more on the keywords themselves rather than even the amount of marketing and promotion put into the site. You could have one really good keyword for a site that does better than 100 keywords you rank well for on another site.
I'm getting 5 visitors per month from google, its frustrating. I'm using very popular keywords for my general link directory.
I'm getting between 1000 - 1500 visitors from google per month, for lots of different keywords but the most popular ones are related to Firefox and Mail Servers. site btw is www.adamsdvds.co.uk (tutorial site)