Martin is closer than the other advice you have been given.
However, you must allow for the type of cleaning your client wants, as well as collecting cups, plates etc, and of course loading/unloading dishwasher if required.
Is the office on two or more floors, this will add extra time!
Three staff? Is the office currently staffed like this? Are you going to TUPE them over?
If none of the above apply, are you confident that you can find high quality, reliable staff to work just one hour a day?
Regards,
Rob