On reddit somebody said while you have the game running, the timer drains. So it's 6 hours of "play time". Not confirmed though?
Yeah I am pretty much going to provide zero confirmation by saying this (I havent been following the EA Access stuff), but I have to assume its timer based. It just has to be. I cant imagine they would count down after your first launch and force you to experience all 6 hours right then and there or lose them.