90 days since last view, or 1 year if you can afford to be generous. For "slow burn" projects with other people checklists easily go longer than a week without being looked at.

That's a great question! And fortunately, one where you can use human behavior as a guide: if someone is actively using a checklist it seems reasonable to presume they're at least accessing it once a day. So as a quick first-pass, double that time period to two days. Also start collecting usage metrics and mine those in the future to further refine this number.

User initiated; maybe some arbitrarily long duration measured in years.