You can leave when your banner goes grey or teammates are afk. Since adding reporting for abandoning the game I report everyone (even in casual). I barely had any complete team with those weekend warriors that storm game today.
It's a TEAM game, you can't do whatever YOU like, because there are 2 other HUMAN BEINGS in trios and your actions reflect on them. You want to quit whenever you want? Turn off "fill squad" option and play alone. You want to have team? Then understand consequences of playing in team.
If this is how we play now, reporting for every little thing, I guess I'll have to dust off the sabotage/inting reports every other game, when some idiot without a R2 button, drops with half the lobby and flies us off to certain death.
