The surface gap plugs simply will not wear out in a reasonable time. They will, however, foul. When the engine becomes hard to start, or will not start, or stalls when put into gear, the first thing to do is remove the plugs and clean them. Depending upon usage, the plugs can be cleaned for years before they need replacement.
They ARE worn out when the center electrode is worn below the level of the porcelain insulator.
That said, If you get the whole season without problems, clean the plugs--or replace them if it makes you feel better spending money--as part of the winterizing maintenance.
If your engine has the Mercury ignition, plugs may not last quite as long as with the older Prestolite ignition since the Mercury is "hotter." However, even with that, a yearly change could be considered overkill.
Remember also: Sometimes (rarely) a used or even a new plug simply will not fire under compression--for no apparent reason. If that is the case, you have no choice but to change plugs no matter what their condition..