Smitty and the other's are correct.
Some people will claim it's 3 months of revenue, other 6, some 9-12. All of it's subjective to so many factors, some of the biggest being how much they will pay, and what you are willing to let it go for.
There are a lot of factors at play, and unfortunately, you do not have a simple formula. Which is good and bad. But I tend to think it's in the seller's favor most of the time.
