Distance vs time for scheduled servicing

We have a Holden Commodore SV6 Series II, which has been serviced by the Holden dealership but only does short trips. As time has gone on, the odometer reading has been far below the nominal kilometres for the scheduled service times. The car is overdue for the 45,000km service but has only 12,292km on the clock. I'm capable of carrying out the service requirements but don't know whether to use the monthly time frame or the kilometres travelled. We also have a Ford XR6 2006 which does some long trips but I service it at 10,000km intervals.