I am looking for a distribution graph of power outages. I am assuming the graph would show that the lengths are positively skewed. From this I would be able to determine how long to have my computers run on backup batteries before automatically turning off.
Ask your local regulatory agency. They will probably have that information.
However, the usual practice for anything but critical servers is to decide how long you will need to finish what you were working on, save it, and shut down cleanly.
If you really want to run for many hours, the simplest answer is just to use a laptop machine. They are designed for power efficiency and to run for hours off their internal batteries, and effectively have their own UPS built in.