Andy this should answer your question.
This is straight from GM Service Information.
Fuel Filter monitor...
"Starting in 2005 on vehicles equipped with the 6.6L Diesel engine have a new DIC message "Change Fuel Filter". Information about this message and how to reset it can be found on page 3-8 of the Diesel Supplement Owners Manual or SI Document ID # 1553021. The DIC will display "Fuel Filter Life Percent" on trucks equipped with RPO UK3 (Steering Wheel Controls). This feature will provide the percentage of fuel filter life remaining. The fuel filter life is based strictly off mileage. Once reset to 100 Percent it takes 15,000 miles for it to count down to 0 Percent and display "Change Fuel Filter" on the DIC."
As for the oil life monitor...
"When the system has calculated that oil life has been diminished, it will indicate that an oil change is necessary. A CHANGE ENGINE OIL SOON message will come ON. Change the engine oil as soon as possible within the next 1 000 km (600 miles). It is possible that, if driving under the best conditions, the oil life system may not indicate that an oil change is necessary for over a year. However, the engine oil and filter must be changed at least once a year and at this time the system must be reset.
If the system is ever reset accidentally, change the engine oil at 5 000 km (3,000 miles) since last oil change."
"Engine Oil Life
Engine Oil Life percentage values are based on class 2 messages to the IPC. The IPC requests this value when Engine Oil Life is selected on the DIC.
Engine Oil Life is capable of displaying values from 0-100 percent and the OIL LIFE RESET driver warning will be displayed.
Pressing the Select button for longer than 5 seconds while Engine Oil Life is displayed on the DIC the value will reset to 100 percent.
Display appears as ENGINE OIL LIFE: XXX%."
So in short:
fuel filter: based on mileage
oil life: based on calculations from driving conditions