How long should a tennis ladder season be?

Six to twelve weeks, and twelve is the ceiling. Shorter than six and half the field never gets three matches in. Longer than twelve and the standings stop feeling live, because a result from four months ago is still holding somebody's rung. Publish the start and end dates before anybody joins.

The other option is to run the ladder open-ended, with no end date. That works for a group that plays all year, but it puts the whole weight on inactivity decay: without a close, nothing else clears out a rung somebody stopped defending. Pick open-ended for continuity, a fixed season for a finish line and a playoff. Season rules.