The percentage you motor not only depends on your schedule, it also depends on your "speed floor". Whether you like the term or not (I coined the term), the "speed floor" is the point at which you will turn on the engines if you go slower. For most people the speed floor will change if your cruising speed under power is different (in a different boat) and whether you hit the speed floor may also be affected by how well your boat can point and sail in light air.

Our boat cruises at about 8k under power so our speed floor under sail is about 4 or 5, or might be even higher if we're trying to make port on a schedule to beat darkness or weather. With a favorable wind we can sail in "the teens", but because we can't count on that our "planning speed" is 5 knots. Looking back on the trips we did last summer our average speed was closer to 6 or 7, but because we plan lower we feel less pressure to get "there" and we're more likely to sail. Very often we'll change our destination mid-trip based on actual conditions and wind direction.