Fvc in Medicine is taught time independent…but in physiology it is taught to be of 6sec…
I think 6 seconds is the maximum time a person can be without naturally triggering inspiration (as I remember the respiratory rate trigger settings in a ventilator we keep at 10/min).
Hence giving the person the maximum time they can without having the urge to inspire due to inspiration neural trigger. But that is not time dependent as the exact time is not measured. 6 seconds is sufficient time to exhale all the air (except of course the residual volume).