Sigh,that is a bit tricky.
I allways plan to measure the time,but often forget it.
Like most people do,I sculpt several models at one time.
I even paint and sculpt together.There're times you have to wait for the putty to harden,so you do something else.
Beside that the TV runs in the background and allthough its more a kind of radio for me,sometimes 'Six Feet Under',Monty Python and the like can be very distracting for me.
Its a hobby for me,pure recreation.I don't care about the time gone.
But I guess pure worktime about 5 hours.