Matt,
This is classic Calculus. It's been over 25 years since I took it, but I was able to refresh my memory with a "Calculus for Dummies" book.
Feedrate is a function of RPM, which is a function of the radius (and CSS) when facing.
Time (min/inch) is the inverse of feedrate (in/min)
So to make a long story short, here is the general equation:
F=feedrate in inches per revolution
R1=starting radius in inches (larger radius)
R2=ending radius in inches (smaller radius)
CSS=constant surface speed in feet per minute
PI=PI
total time in minutes = (PI / ( F X CSS X 12)) X (R1 ^ 2 - R2 ^2)
Plug your numbers into this equation and you'll get 8.3776 minutes.
The answer is exact. I rounded it to 4 decimal places.
If you clamp your spindle speed before reaching maximum rpm's, (at 4 inches) it will take longer.
Diplomacy is the art of saying "Nice doggie" until you can find a rock. - Will Rogers