This is a difficult question to answer, primarily because there are different type of sales and different sales agendas. For instance, someone could have a highly desireable name, that if put up for sale at a reasonable price, would sell instantly. While, others may have a name that only makes sense for a certain company.
Ive found that some domains can sit on the shelves for several years before attracting the right buyer, and Ive also had domains that have sold within hours of acquisition. I dont think there is any clear defining timeframe on how long it takes to sell a domain name.
Justin