Ask HN: Rules of thumb for website valuation

The recent Ask HN post asking who has a side project for sale got me to thinking about rules of thumb for valuing websites. One commenter on that post stated a rule of thumb that a website would typically be worth 9-10 months of revenue. This seemed rather low to me. What are some other rules of thumb for website valuation you have heard of, and does the rule change depending on the revenue model, ie, advertising only versus paid subscribers?