That is actually the worst thing that you can do to your engine, as well as to your exhaust system.
If you want to "exercise" the engine, you need to get the engine up to full operating temperature and then to drive for over 30 minutes in order to evaporate all of the accumulated water condensation that has been deposited in the motor oil as a result of all of those short trips.
When you don't evaporate all of that water condensation, it turns acidic over time, and leads to deposits of damaging sludge in the engine.
All of that being said, I would recommend that you not go longer than one year before you change the oil.
Simply "looking clean" is not an accurate indicator of how contaminated or acidic that oil may have become because of the short trips and infrequent oil changes.
Ask yourself this question:
Would you wear the same underwear for several days, simply because it "looked clean"?
Just as good personal hygiene dictates daily changes of your underwear, good engine hygiene dictates much more frequent oil changes than you are contemplating.