Yes, the antivirus scan can be painfully slow. Several days is absolutely unacceptable! What is the size of your computer's hard drive? Do you have many compressed files stored on it? Do you have any external drives or network mapped drives attached?
You can improve the OneCare scan time by excluding any mapped drives and external drives from being scanned during the normal tune-up. In fact, you can exlude just about anything, if desired, though that will compromise your safety somewhat. Another recommendation made often is to exclude folders containing .cab files, zipped files, and exe installers, as well as any image files (ISO, for example) that you may have on the drive. OneCare deep scans compressed files, which can be very time consuming.
-steve