A Supply Drop will fall on a random location every ~1 hour 15 minutes on average, these drops contain between 8 and 16 military items.
The interval between drops is 1 hour + some random amount of minutes between 0 and 30 and the location of the drop position will be announced in the chat box as it starts falling (not when it lands).
The idea of the supply drops is to provide a dynamic loot hotspot that should result in similar battles seen at hotspots around the map when the server restarts. This makes the server restart less significant for looting and ambushing hotspots.