When you say "when", do you mean depending when is the best time or making sure you, as a team, consistently do it? The latter is from my experience the hard part. It's hard to make sure that's part of the process as it's by necessity quite awhile after a feature has been released and no one is actively working on bugs related to it.

We've started to use automated scripts that identify "old" feature flags that are turned on to 100% that are still in the code base and treat them similar to bug tasks.

Define a macro which has the feature name and a future date for when the build should start complaining. For extra points, have two dates: one for generating warnings and another for generating errors. Works like a charm.