Values go stale. Set Remind me to revalue on an item — Every month, Every 3 months, Every 6 months, Every year or a custom number of days — in its form under Notes & tags.
To set it for many items, select them in Holdings and choose Reminder…. See Selecting many items.
Items due appear on the Overview under Due for review and Needs attention, and in Holdings under Needs attention → Due for review. Recording a new value restarts the clock.