It’s really going to depend on the number of CPUs and the amount of Main Memory you have too. You can imagine it’s not going to be short. I think I remember on a 1 CPU 16 GB Power5 box the upgrade took about 3-4 hours total using Virtual CDs. Once I used virtual CDs I will never load them again and wait.
I would plan for 6 hours but think you will be well under that.