I did an upgrade install of Tiger yesterday on my Power Mac, and it only took about 15 minutes or so. When I installed Tiger on my Power Book using a fresh install, it seems like it took around 30 minutes, but I can't remember for sure.
As an aside, people around here seem to have had trouble with upgrade installs, but the one I did yesterday worked just fine. I had some issues with the pro apps, but those were things that were very predictable based on others' experience and a reinstall of the production suite fixed them.