I have a lot more miles touring than brevets, thus my answer is oriented towards touring.
I am not pedaling when approaching a stop sign or stop light. I am not pedaling when going fast enough down a hill that I run out of gearing. Otherwise I am pedaling. And shallow downhills, I pedal down those to if it means that I can carry some of that speed up the other side of a valley.
Based on that, in areas where there are lot of intersections, like communities, maybe 5 to 10 percent of distance is coasting as I approach stop lights or signs. In hilly terrain, maybe 20 percent of distance is coasting down hills. Where it is flat (think Southern Florida), maybe 1 percent is coasting.
In hilly terrain, even if 50 percent of the distance was coasting, that might represent less than 15 percent of the time on the bike, downhills are quicker than steep uphills.
There is no single number.
The graphic is the elevation profile from the day I rode from East to West on Going to the Sun Road, data from my GPS. Maybe 40 percent of the distance was coasting, but the steep uphill took a lot more time than the downhill. The coasting did not take much time at all.
