"All time should really be logged."
I agree - and if you do it that way, you'll find you get a lot fewer components breaking down!
However, when I were a lad and did a lot of pleasure flying, the accepted practice (from the CAA) was to put down two thirds of the start-to-stop time as the flight time in the tech log. Of course, I used a stopwatch, but I found it wasn't that far out.
Phil