Well, that depends on a lot of things.
1. What interface is the enclosure? Is it USB 2.0, FireWire 400, FireWire, 800, eSATA, etc.?
2. If FireWire, is it a good Oxford-based enclosure, or something generic that came with a pre-built hard drive?
3. What make is the actual internal hard drive inside the enclosure? What RPM is it? What size is it?
4. What speed is the internal hard drive inside your computer? What RPM is it, and what size is it?