"Never. Or forever. Car trunks are not water tight let alone air tight. But other things can cause death in a trunk like extreme heat or cold, being in an accident or suffocation due to something being on top of one/not being able to roll due to no room."
Don't forget carbon monoxide poisoning if the exhaust system has any problems, getting trapped if the car goes off the road, and even drowning if the car goes into water because most trunk lids don't have an inside release. Mostly, though, the danger is a collision both because the passengers aren't secured and because most cars are designed with the trunk space as a crumple zone to protect occupants in the car's cabin. I don't know if it qualifies as a Special Snowflake move, but it's certainly a very dumb idea.