We keep a small ledger on the Annfield wall of windows we have actually defended. A grocery app that restocks households: twenty-eight days, because the ritual is a monthly shop with a weekly top-up that should not count as “retained.” A language app used on commutes: seven days, because the product really is a daily habit and pretending otherwise would be costume.
We recanted a fourteen-day window for a Belfast insurance companion in 2024. Fourteen looked “balanced.” Claims and policy changes do not live on a fortnight. The honest window was a calendar month aligned to billing. The graph became uglier. The operations team could finally use it.
Daylight saving still surprises well-paid analysts. If your window is “seven times twenty-four hours” rather than “calendar weeks in Europe/London,” you will invent churn every March. This is not advanced. It is omitted from vendor demos because it is embarrassing.
The Packet Ledger asks students to write the rejected windows down. A board pack that only shows the chosen series is a brochure. The rejected neighbours are how you prove you were not shopping for a prettier slope.