Letting it heat up and run at a decent operating temperature once a week or so will help prolong its life IMHO. Running an engine for only short periods of time is as bad or worse than not running it at all.
Also, the ideal RPM will be at about 80% of the max RPM. 1800 - 2000 is probably under revving for a 3 cyl. Yanmar. You'll want to check the max RPM for your motor, but I would guess 2400 - 2500 would be about your target cruising RPM.