I've got two 500GB SATA II drives (10K RPM, 3.0GB/sec) setup as RAID 1.
I was a little surprised (impressed) that it took about 2 hours to rebuild the mirror once the failed drive was replaced.
The drive that failed was not old at all, probably 6 months. BUT, we abuse hard drives pretty good. A lot of IO taking place. I have to defrag daily (diskeeper btw is a great background defrag product )