How long does a crate drop take in Grand Theft Auto V?

While playing Grand Theft Auto V online players are occasionally notified of a crate drop. A green circle appears on the map and a flare appears in the game world.

I have responded to a few of these crate drops notifications but am confused by how long it takes between the notification and the crate actually appearing.

Recently I spent over 15 minutes at the flare site shooting any player who came close (sorry guys, nothing personal) but the crate still didn't appear. I have come across two dropped crates, so I know they do drop at some point.

Is there a set time limit, a random time limit, or is the crate drop triggered by something else (like having a certain number of players occupying the drop zone)?

I'm pretty sure the normal crates spawn randomly when it comes to time, and if you go to the drop impound right after it shows on the map the crate will never spawn, I think it's a bug because one time I did that and I waited 3 real life hours, and one time when the inpound spawned I waited one ingame hour and the crate spawned.

I'm not sure about the special crates because I only saw an ingame notice about them only 2 times, but I think it's something like 1 day or 1 week in real life.

And when you see a special crate notice and it will say "special crate drop in whatever hours" it will mean ingame hours not real life hours.

hope this helped. :)