That's one way to do it…Not to be overly simple, but won't pretty much all his cameras show a "record time" length in the viewfinder, based on codec and resolution selected?
As far as the P2 times:
DVCProHD and AVC-Intra 100 = approx. 1 min/GB
DVCPro50 and AVC-Intra 50 = approx. 2 min/GB
DVCPro = approx. 4 min/GB
And as others have said, it also depends on frame rates, etc.
In my 2700 a 64GB card gives me 70-72* mins at 60P, and around 180+ at 24P native.
*When my cards were new the cam would indicate 72, as they have gotten age and cycles on them, they have reduced to 70.