It claims that the generator should be run for 30 minutes every month, loaded to at least one third of the rated capacity. So testing every month is exactly what you want to do.
Also don't forget to check the fuel tanks. With the rise in fuel prices the past couple of years, theft of diesel from backup generators has become more common.
Here's a doc from cummins, a generator mfgr: http://www.cumminspower.com/www/literature/technicalpapers/P...
It claims that the generator should be run for 30 minutes every month, loaded to at least one third of the rated capacity. So testing every month is exactly what you want to do.