I think realistically only those purists (with no schedule - like the Pardeys) sail ALL the time. We recently spent 56 days cruising the BC West Coast, Georgia Strait, Jervis Inlet and Desolation sound. Many of these areas are typically wind vacuums in the summer. In 41 days of travelling we racked up 120 hrs on the engine. A few of those hours involved idling looking for decent anchorages, moorings etc. Of the 1056 NM we covered between the end of June and late August we probably motored 600 due primarily to lack of wind. We averaged 25-30 miles/day with two 90 and one 110 NM days out to and back from the West Coast. We enjoyed many 4-6 hour spinnaker runs, but suffered just as many calm days motoring. Our sail/motor threshold was a VMG of less than 2 knots (depending on the distance remaining) The first mate is not into night sailing. Period. So the longer days necessarily included motoring. The 17 hours we put into the 110 mile day was half motoring in calm/fog and the last half blasting downwind in 20 - 25 knots.
We never motor downwind in over 10 knots of breeze, and will most often sail upwind if the breeze is over 6-8 knots unless long distances are involved.
The last two days of our trip, reluctant to head home, we beat into a SE wind that varied from 5 - 20 - 5 knots over the two days, against a knot and a half of tide. It took us 40 miles sailing to cover 28 miles distance one day, but it was sunny, windy and thoroughly enjoyable. Did I mention no one really wanted to get home?