An example:
A receptionist does not necessarily need focus time, and is interrupt driven, so their work should not stretch to more than 5 minute intervals; if something takes more than 5 minutes it must be filed for someone else.
A manager manages their time in 30 minute increments, anything that can't reasonably be done inside a meeting is unfortunately unlikely to happen. This is stupid, but it's unfortunately true; you might sometimes experience other situations but I think that's not the common case.
A programmer (or, researcher, etc;etc) books time in half-day increments. If you must have a meeting with programmers, it's often better to book events in serial but only consume half a day.
I've been practising this methodology for 8 or so months in my org and it does seem to hold true in the general case, there are some minor exceptions but most people do seem to he happy.
EDIT: It was the "Makers Schedule" by PG; Serves me right not to read the comments[0] before commenting.