I always thought that the despawn time was 2-5 minutes, but I found out about the Skyshard farm in which you kill monsters for 25 minutes and then loot them since the Skyshard is a purple item and stays on the body for 30 minutes. I would like to ask if this does apply to every purple like the dune scavenger?
If you do not leave the area (distance wise or zone wise) far enough for it to de-spawn the item and body will remain lootable to you until you do so.
If you do leave, the item will show up in your mailbox. WoW has special handling of higher rarity items so things like purples do not get lost by accident.