The dyno runs for 15 seconds. I use peak hold so I can record the final value. When I clamp for efficiency, I stop the dyno before it is done reading so I can get the clamp as accurate as possible.
The only problem with this is since the dyno and clamp are separate tools, you cannot guarantee 100% accuracy. The only way for that reading to be 100% accurate would be for the dyno to have a built in dcA clamp that measures at precisely the same millisecond as the wattage reading.