Let me share my experience with you all. I'm a full-time working mama but I didn't express milk in office. My schedule is:
6:30am direct feed my bb + express milk
7:00-7:30pm express milk
11:00pm express milk
Though the volume is not that large (~2Xoz) but still have enough to feed my bb by bottles in day time when I'm working.
Actually, I wanna express milk during office hours also but I'm reluctant to do as:
1. no clean place available (toilet is dirty =P)
2. office work is already busy (busy also when home with my bb), wanna have some time to take a rest, get some nice food, some chat with colleagues and totally relax myself.
That's my own opinion. Of course, express more frequently would be better ideally!