Penalty when leaving after a team mate leaving

I think it is good that people get punished for dodging or leaving game within 5 minutes or something. But I just qued a game where my team mate left during the loading screen (in a 4v4 ranked). So I immediately left when the game started and was not allowed to join que. Not sure if this would reach anyone it may concern but just want to put it out there.

Additionally it would be good to include a loss prevention which a lot of other games have. If your team mate leaves before 5 mins or something then you can also leave without dropping ELO.

Aside from that. How lang do you have to stay in the game to avoid a timer?