Here's what I got from the service manual:
Oil Life Warning (CHANGE OIL SOON or OIL CHANGE REQUIRED)
One of these warning messages is displayed when the engine oil life remaining is five percent or less.
- When oil life left is between five percent and zero percent, the "CHANGE OIL SOON" message will be displayed.
- When oil life left reaches zero percent, the "OIL CHANGE REQUIRED" message will be displayed.
The message center indicator will indicate the percent of oil life left during System Check. This percentage is based on the driver's driving history and the time since the last oil change. In order to ensure accurate oil life left indications, the driver should only perform the OIL CHANGE RESET procedure, as described in the Owner Guide, after an oil change.
To calculate the percentage of oil life left, the oil life feature uses:
-- The oil temperature sensor input
-- The tachometer input from the ignition control module (ICM) (12K072) (ICM)
-- The vehicle speed sensor (VSS) input and
-- Clock time (maintained internally by the message center)
- The oil life will decrease from 100 percent to 0 percent in no more than 8,000 km (5,000 miles) or 6 months. It will reach zero percent sooner under different driving conditions.
- The percentage of oil life left is the second display in the system check sequence.
- When oil life left is between five percent and zero percent, the "CHANGE OIL SOON" message will be displayed.
- When oil life left reaches zero percent, the "OIL CHANGE REQUIRED" message will be displayed.
I guess it's based on a 5,000-mile interval. But I set mine at 80% and I always change my oil at around 5000 miles so I dont' know.