It's simple - the last time a plugin was "used" was the last time JIRA was running.
The question doesn't actually make any sense for many add-ons. If it's providing a field for example, then as long as there's data for one or more issues, it's in use.
There are some add-ons for which you could say things like "this report/export has not been used by a user for X", but there's no way to track that in JIRA. There's no "history of usage" for any add-ons, even the ones where it could be done (which isn't that many)
I assume you are trying to reduce your add-on usage (and hence bill). The only way to do this is evaluate what you installed an add-on for and ask your users if they still need it. The harshest approach (I'd only do this after asking the users and having no-one admit to using something) is to simply disable an add-on and see if anyone yells at you.
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