Depends how fast the computer is. But, yes, it takes forever. And besides it's worthless junk. (If you want any A-V, get Sophos instead.) In order to uninstall, here's where all the files are. Basically just drag the app, underlined below, to the Trash. Do the others if you want, but they aren't very important.
Get rid of the tracking cookies. They are used to profile and track your browsing history. While they are privacy invading, by calling them spyware, MacScan is being a little dramatic in trying to sell you its crap. And in the future, for whatever browser you use, don't allow third-party cookies.
To prevent tracking, get Ghostery. In addition to having Ghostery and forbidding third-party cookies, I clear out all cookies from one browsing session to another. If you always do that, you won't have any tracking cookies to worry about, so you won't need MacScan to find them for you. Btw, MacScan finds the tracking cookies in the first few minutes of scanning; if you want to use it for that, then that's all the time you need to run it for. But, as I said, you won't have any tracking cookies around if you just remove all cookies and don't allow third-party cookies. As soon as you visit a site that needs them, you'll just get new ones. No problem.
Read all about cookies here.