There is not all that much regulation about that stuff to be honest. You need a rest time before your flight duty. Office hours would not be considered a rest time so you can't have office hours in that rest time. However, there is the exception that office hours directly followed by a flight duty are of course possible, but the office hours are then considered flight duty time, which of course limits the extend of the following flight duty.
Outside of flight duty time and rest times there is no regulation about office hours except the usual local working time directives which as the minimum in EU countries have to follow the EU working time directive.
There might be regulations about that in local rules and/or CLAs. In my company office days will be planned with a blanket eight hours duty time (not flight duty though), no matter how long one has to work. I believe our CBT days are considered 12 hours duty though, to force a 14 hour rest time before we have the first simulator duty on the day after.