On an iBook G4, with about 35 megs of total files (about 20-25 in the Home dir), it took less than a half for Spotlight to index completely, IIRC. No, this isn't normal. After the first index, unless you force re-indexing, it should not do a full re-index, even if you reboot and/or repair permissions, update prebindings, update software, etc.
I don't have any help for you, sadly, but it doesn't sound right. Even if you have a much larger hard drive (which will also doubtless be much faster than mine!), the reboot and re-index part is not right.
EDIT: You don't have anti-virus software installed, do you? If you do, uninstall it, and put it on the ground and jump up and down on top of it, and then light it on fire. Some of the packages, esp. Virex, can really screw Spotlight / Tiger up.