Whatever a buyer is willing to pay.
On a more serious note, it depends on many things. The life expectancy of the site. Some sites have limited lifespan due to the nature of their income such as let's say something like a specific title of a video game, a specific olympics year, or a current event. These are worth a lot less.
If it is more permanent then the method of traffic is then important. If it's viral traffic it's worth less, but if it's steady search engine results that's better. Consistent type in traffic would be best because it signifies repeat visitors.
I personally try to buy at 6 months or less if possible, but then again I do most of my buying through contacting them or on forums not meant to sell websites. This keeps my competition with serious buyers down to a minimum and allows me to get a better price, plus they don't know what it's worth usually.
I sell at 12 months because my sites are in high demand, especially to the right buyer. 12 months takes a bit of negotiating, but it can happen.