Frank,
A good standby run schedule would consist of a test-run once a month with load. Minimum time would be long enough to get to full operating temperatures (with load). This is a fairly standard operational readiness regime. Your own situation and specific requirements would trump any arbitrary schedule. The main points are prevention of stale fuel sitting inside the machine, drying out of seals and gaskets, condensation remaining in place on any internal or external components, any homesteaders in the way of rodents and insects getting too cozy in or on your machine, dead batteries and the like.
dieselgman