A mechanical watch is a small machine that needs looking after, and most of them arrive with no paperwork at all. Buy one second hand and you are lucky to get the box. The service history, if there ever was one, stayed with the last owner.
Two schedules, not one
A full service and a pressure test are different jobs on different clocks. The movement wants cleaning and oiling every several years, depending on the watch. The seals want testing more often if you ever wear it near water. Treat them as one date and one of them is always overdue.
The interval depends on the watch
A vintage Seiko 6309 and a modern sealed diver do not want the same care. An old movement behind tired seals wants attention sooner. A newer watch with a fresh service can go longer. The sensible interval is the one that fits that particular piece, counted from the last time the work was actually done.
Write down who did it and what it cost
When you come to sell, a documented service history is worth real money. Anyone paying for a vintage piece wants to know it was serviced, when, by whom, and what was replaced. A receipt in a drawer is half of that. A record kept against the watch is all of it.
Winders add one more thing to track
Run a winder and there is also the small daily question of which watch is in which slot and how each one is keeping time. Easy with two watches. Less easy with eight.
WatchLog is an Android app that keeps a service record for every watch in the collection: reference and serial, the work done and by whom, what it cost, how it is running now and which winder slot it sits in. It tracks the service and pressure test intervals separately, and the whole collection prints as a PDF.