Ah ! that clears the water!
There are a number of service intervals, depending on who's generator is installed and the fuel used.
LP burns the cleanest so the oil does not get as dirty, but one still needs to change it once a year along with the filter per the hr.meter and change air filter according to the hr information in the owners manual.
With Gas and Diesel units one also needs to change the oil, air and fuel filters according to the manuals.
The larger units need to have the coolant changed on a regular bases.
Most generators have set maintenance intervals, like 250 hrs, 500hrs and 1000hrs. the 500 and 1000 hr repair intervals can require removal of the larger units to replace the water pump belt and possible cab. work, spark plugs and wires.
Diesel units would include injectors, glow plugs and some other items.
Hope this information gives you a good baseline.