How do you know when to remove a feature flag?

But once a feature has been enabled, how do you know if you can remove its flag?

A simple suggestion I've heard is to add a "Remove when..." comment above where the flag is used and detail what conditions need to be in place before the flag can be removed.

It might be a period of time or after another feature has been shipped, but it will give some clarity when you see the flag in the code to if or when it can be removed.