The owners manual list the full service schedule. Volvo's should
be maintained "by the book" for long life (they are not at all
drive and ignore cars). The light can be set to go off at different
intervals (so it may be set every 7K miles or every 15K miles, the
dealer can actually change it). The light may also go off at 6 months or
12 months (it can be triggered by time or milage). Cars that are driven low
miles may need oil changes every 6 months so the light may come on at an
odd milage. The exact service light timing/milage schedule used probably
varies by model year, too.

You should look at the service schedule in the manual and determine what has
and has not been done and then get it taken care of. This is really
important. As an example, brake pads last about 30 to 35K miles. But,
the brake pads do not give any indication if they are worn (some pads
in other cars start to squeek). The pads have to be visually inspected.
This is the sort of thing that is checked at a regular service.