Most of the service schedule is based around mileage.
6k service is plugs, oil, filters, valves etc.
12k as the 6k plus fork oil, brake/clutch fluids, coolant and belts.
6k service is generally termed the annual service and 12k is two years.
Even if you don't do 12k in two years, it's worth having the fluids changed as they degrade over time.
You can get an idea of what's involved by checking your service book. It lists all the operations by mileage.
btw.........IMO it's worth having the belts changed at 6k. Cheap insurance
