Uhmm its almost that it can stay opened and closed for as long as it can. It can stay open as long as you want it 2 but if you have it closed it will only stay 180 seconds and then a gravlift will spawn. but if you pick up the b again it will reclose and then you have anoth 180 seconds
Seen it before and done it before... Also if yo like the dumpster as it should and have the wall above it when the Dumpster goes up a Warthog is capable of getting through, you can repace the dumpster with a crate aswell.