How do I calculate CFM for a structure?
The "minutes per air change" is a design value chosen for the type of space and use that space has.  In common situations, that number is ignored and simple rules of thumb for air flow are used instead.  (Living rooms don't vary greatly in size, and the target numbers usually have a magnitude range of 2 to 4 anyway.)

However, a common design for bedroom space is 1 to 4 air changes per hour (15 to 60 minutes per air change).  Living room space which is uncommonly occupied can be once per hour (60 minutes per air change), whereas a crowded smoky bar is comfortable at 15 to 30 air changes per hour (2 to 4 minutes per air change).

There is an extensive table here mostly covering commercial spaces.