If our flight leaves at 5:15am, how early do we have to be at the airport?

If check a bag-4-4:15. The airline counter usually don't open until 4am

For my last flight at 5:15 a.m. I was at the airport in the security line by 4:20. I would have missed my flight had the airline not held the plane because the TSA checkpoint is so slow.

They suggest you get there 2 hours early. If they're not open you can familiarize yourself with your surroundings and prepare for your flight!! Better early than late!!