My Onan generator manual suggests exercising the generator for 2 hours once a month under 50% load.
Copied from the manual:
"Exercise the genset at least 2 hours each month if use is infrequent. Run the genset at approximately 1/2 rated power. See LOADING THE GENSET (Page 2-2). A single two hour exercise period is better than several shorter periods. Exercising a genset drives off moisture, re-lubricates the engine, replaces stale fuel in fuel lines and carburetor and removes oxides from electrical contacts and generator slip rings. The result is better starting, more reliable operation and longer engine life
My generator is a Onan 4KW. For loading the generator I run one 15,000 BTU roof top A/C unit + whatever misc small loads I might be using at the time.
How often (or not) do you exercise your generator and how do you load it?