Oh, they are busy... Let me give you an insight to my professor's and my own schedule (I'm lecturer, not yet regular professor as I am still lacking the habilitation thesis).
We have a short meeting every working day at 7.30 to organise the daily business and schedule big meetings for big issues.
After that there is always enough to do: Those who lecture always have something to prepare: Writing or correcting scripts, correcting theseis and exams, preparing slides, organising participants for congresses (or participate themselves) and prepare talks for such events. Many professors and lecturers also hold talks outside of university, so there's also this stuff to prepare.
Then there are commitees. Mine has to organise a major congress in March 2015, we are already very busy now: Contacting possible participants, reading through all the literature the participants wrote on the topic, preparing our own papers and talks in response to it, etc.
My working day starts at 7.30 and normally doesn't end before 7 to 8PM. Just yesterday I attended a congress from another commitee and including the dinner (where you also keep discussing) I spend my saturday at university from 7 in the morning until 11pm...