before Green Line there's enough time to get up the stairs/escalator. Going North to the airport is easy because you're transferring from FrontRunner which runs every thirty minutes or hourly to Green Line which is every 15 minutes. Going South can leave you sitting at North Temple for a while.
Buy your UTA ticket using the "Transit" app. If you can, schedule your UTA ride(s) as if your FrontRunner will break down so if it does you won't miss your flight. Experience.
Happy travels.