Economics 101 optimal is where the supply curve meets the demand curve. For most STR's its around 60-70% occupancy. Lower for lux properties and higher for econ properties.
Andrew's 60 to 70% is a good sanity check, and the way I'd say it to a newer host is that occupancy is only good news if you know what rate you bought it at. One of my properties hit 100% occupancy this month and that number means nothing by itself, it only counts because the rate went up too. Last year was rough for me so instead of dropping price I did soft upgrades to the listings, and I'm charging about 25 to 30% more now than the year before on the same properties. If your calendar fills easy, thats usually not a win, thats the market telling you you're the cheap option.