We had a town-wide disaster in February and ran on generator for over 8 hours. My whole building was shut down, so we moved to the command center. Luckily (or not luck so much as good DR planning) our critical servers were in our primary server room deep in the bowels of a mountain on a different power grid.

We would have been up sooner but the power company thought that getting residential areas on power were more important than the hospital, EMS, police station... at least until someone figured out what they were doing and "suggested" they refocus their efforts.