I change the main filter around every 10k miles, it holds about 1 qt.
The bypass filter hasn't ever been changed, was installed with about 10k on the oil... it's been in place for about 32,000 miles.
I haven't kept track of how much oil I have added. I know coming up the ALCAN in 2008 I had the engine breather freeze up at -40* and it dumped about a gallon of oil out the dipstick tube.
Add that with the filter changes, and top offs for minor leaks and normal consumption, I'd guess I have close to a complete oil change over the course of the ~4 years that oil has been in the truck.
I'm sure I have been less than "scientific" with all this, I really just expected to change the oil out when a sample tested came back being bad.
Yes 15 qts/3.5 gals is what the truck holds with both filters changed.