The office complex I work at has two generator systems: one for the elevators, and one for lighting. Both systems are setup with automatic scheduled tests. The elevator system is run every Friday for about 2 hours. The other one comes on during a weekend once a month (I think... ), and runs for a few hours also.

We do not have anything super critical, but it's all automatic. The only time I think they are really used is during the tests, and Demand Reduction orders issued by the electric company. The power rarely goes out at our complex.