[elhigh]

Uhh, well. I don't have any way of recording that, but it could be done:
An engine hour timer is easy to be had, not expensive, and hooking them up isn't hard either - wire one up to the ACC position, so it's on whether the engine is or not, and the other to the IGN ON position, so it's recording only while the engine is running. Compute the difference, voila: done.

There's other ways to do it, that's the first one that popped into my head.

If I had to guess, I'd say as much as 10% of my commute can be done Engine Off. I track how many miles I cover gliding, it tends to be about 10% of the total distance.