A session is not a philosophical puzzle. It is a cut you make in time so that a sequence of events can be talked about as one sitting. Vendors offer a thirty-minute timeout because it is a round number that offends few people and matches few lives.
Riders between jobs unlock a grocery app, stare at a list, take a call, return. A thirty-minute glue turns two lunches into one loyal shopper. A five-minute window splits a bedtime story when a parent answers the door. Neither default is “wrong.” Unwritten defaults are.
Watch the phone, not the SDK
Sit with five recordings or five diary days. Note backgrounding, lock screen, and the job. A banking glance is short. A language lesson is long and interrupted. Write two candidate windows. Reconstruct a week of events both ways. The better window is the one that changes fewer product sentences you already believe — or, more honestly, the one that makes a known lie visible.
Backgrounding
iOS and Android do not agree about what “background” means for your SDK. If a navigation app stays warm with location, a session that dies on background is a fiction. If a reader suspends, a session that lives for half an hour in a pocket is also a fiction. Put the rule in the taxonomy: “session ends on background after N seconds unless playback is active.” Engineers can implement a sentence. They cannot implement a vibe.
Household devices
A session on a kitchen tablet is not a person. Identity and sessionization are cousins. If you have not written the identity memo, do not spend a week polishing timeouts. You will attribute a teenager’s snacks to the adult who pays. We have seen that chart in the Midlands. The board preferred the old green arrows until the limitation was printed.
Publish the window
Once chosen, the window is part of the ledger, not a hidden setting. When you change it, cohort charts from before the change are a different document. Label them. Do not “fix history” in silence so that a quarter looks smoother.