There is not a standard answer. It could be one month or five years.

For the seller it's not just about what the site itself is worth. That revenue stream is gone forever. Do they depend on that money? Does the buyout figure cover the development time of a new site that will match the lost income? Maybe the site was expendable in the first place. Especially if they own 10 sites and work is being spread too thin. In that case the sale price was free money since work can just be shifted to the other sites.

The buyer has a lot more to look at than just sales figures also. They are only relevant if the site will continue to be operated the exact same way it was under the previous owner. It's about what the buyer thinks he can do with the site in the future.

I once bought a blog for two years worth of income. I recognized that the operation of the site was a joke, and that I could get an ROI very quickly on it's level of traffic.

So the key as a seller is to find someone who thinks they can run the site better than you ever did.