> How does this compare with the experience of others?

Hmmm. A quick calculation shows:
My biggest database is 1/74 of the size of yours.
I need ~5 Minutes to backup a freshly restored database, backup takes
longer if the restore is longer ago (up to 6.5 minutes).

5 * 74 = 370 minutes == 6 hours and 10 minutes.

certainly not any academical method to do a comparison, but
surprisingly close to your result :-)

Mine is on hardware of 2005, RAID, Dual Xeon 1GHz, Server2003 R2