Over the course of about 2 weeks my raid5 system broke twice requiring new disks and a rebuild.
The controller was a Highpoint RocketRaid 1640 SATA with 4x Seagate 250GB 7200RPM SATAII drives.

Rebuild time was as follows:
Build 1:
raid 5 rebuild started approx 4:25PM
5:27PM – 21%
6:24PM – 40%
7:04PM – 53%
7:28PM – 61%
8:35PM – 82%
9:25PM – 99%
9:27PM – 100%

Build 2:
raid5 rebuild started 11:51AM
2:13PM – 47%
2:51PM – 58%
3:44PM – 76%
4:25PM – 90%
4:55PM – 100%
I couldn’t find any reliable information on an estimated rebuild time, so theres my experience.