It's really hard to say exactly because there are so many factors that go into buying and selling domain names. I think the most important thing is the domain name itself and whether or not it would be in demand. Then, a lot would depend on how much you were asking for it. If it is priced too high, it could be on the market a long while without ever selling. If it's a great name and you price it right, it could sell very quickly.
Is there a website at the domain or is it just a vacant domain name? If there is an operating website that already has traffic to it, it could add a lot of value and fetch a higher price.
One major factor besides price and popularity though is how you intend to market the name. You'll need to get it out there so it can be seen. You can market it and try and sell it at a set price or you can put it in a domain name auction. I think the auctions get more exposure, so that might be the way to go if you want to sell quickly.
There are several websites that deal in the buying and selling of domain names. Sedo.com claims to be the most popular and widely used, but you can also use godaddy.com or afternic.com. I'm sure there are others also. I know Sedo has a domain name appraisal service that will help you determine your domain name's value for a fee.
If you are just starting out and want to be a domain name "flipper" or reseller, I suggest you buy a book or two on the subject before you invest much money. There are several good ones available in ebook or print versions on amazon. Good luck.