I wouldn't worry about the exact balance of traffic and ad revenue you can reach until after you launch. Because frankly, until you launch, you have no idea how your traffic will look like.
First of all, the most likely outcome is that you don't get much traffic: your site is just not that exciting for users. So worries about ad revenue becomes moot.
Second of all, even if your traffic starts growing, you will probably pivot and tweak things many times to hit the sweet spot. Think of Justin.tv giving up on random videos and focusing on gaming. There is no way they would have known until it happened to them.
In the end, once your traffic looks good, there will be plenty of people (investors and consultants) happy to show you how to make money with it. The challenge is to get the traffic.
In the meantime, you can play with some basic math. 1000 visitors can net you one dollar (estimate can be off by 10X either way). Estimate your hosting costs and see if it's sustainable.