There's no way to reliably track when a ticket is resolved. Two examples...

1. User writes in -> Agent responds -> User writes back to say thank you -> If the agent just says you're welcome, suddenly it doesn't count as a first contact resolution, even though it was.

2. User writes in -> Agent responds -> They go back and forth and resolve the issue in 2 days. User writes in about separate issue 2 months later on the same thread. So now it looks like it took 2+ months for the agent to resolve the issue.