Would depend on the history of the site, if it is a high quality sites and has had stable traffic for a long time I think it could be years, if it is some type of MFA site then it might be months.
If it there were any legal issues such as brand infringement, trademark infringement it would dramatically decrease the price of the domain for me. Or anything that would pose a risk to the earnings.
Hahaha wouldn't say an expert, but was unfortunate enough to lose a couple of domains through UDRP. Shattering. Don't register anything with the term "Lego" is all i can say.
Ok - to make it all clearer so people can be brave enough to put a price on it.
Site is 4 years old.
Has held #1 ranking for 2 years (exact match)
Has consistently earnt $3k a month from Adsense for 12 months
Want to sell to acquire new name
No trademark issues
Huge traffic (again, consistent)
It's a .au domain
This of course is an example, there is no actual site here for sale.