Full Load vs Partial Load

The load is based on the design temperature of your location and the size of the equipment. If your outside design temperature is 0 degrees, that would be the only time the boiler is operating at its theoretical full load. If the boiler is oversized it may never operate at full load.
There are cycle timers to operate a certain amount of cycles based on outside temperature and building requirements. The key to operating all equipment, for maximum efficiency, regardless of load, is to operate them at their maximum rating. Maximizing differentials (ON/Off) is the key to minimizing short cycling. All equipment has a certain mass that needs to be heated. Burners heat heat exchangers, not buildings. Trying to operate at less than maximum rating greatly reduces efficiency and can cause excess wear on equipment.

Depends a little on what you mean by "full load". By definition an on/off appliance -- like almost all steam boilers we see -- is either on, and operating at full load for whatever burner it has in it, or off. Partial load isn't meaningful. For a modulating burner, it is. On the other hand, if you are thinking of the system as a whole, where the burner is effectively pulse modulating (turning on and off as required by a thermostat, pressure control, or what have you), then -- almost all the time. And I would say that considering the system as a whole, you're looking at closer to 99% of the time, rather than just 90%.