How soon after buying a watch should I put my watch in for a service?
From a care point of view, regular servicing will result in less risk of serious harm.
If you are trusting the watch for wear during diving then it is recommended to at least have a reseal and pressure test each year.
If wanting to keep the cost of repairs to a minimum then only addressing issues as they appear will cost less over time, as long as you are wary of:
Not allowing dust or water to enter the watch, that would harm delicate parts such as the dial.
Not leaving an automatic watch so long that the rotor bridge is damaged from lack of oil.
Other than that, in general, most are safe to wait until the watch has failed to keep time as accurately as they require.
Though that is purely from a cost point of view.
So how often do most people get their watch serviced?
For quartz watches:
As long as the case is resealed with every battery replacement, a full service should only be necessary every 10-20 years with a high quality movement.
For mechanical watches:
It really depends on the wearer and the accuracy they require, also the calibre of movement plays a big factor.
Generally most mechanical watches will need serviced some time between 5 and 10 years from the last service.