If you want something more specific you are probably going to have to actually time it. Bosses' "essence" will last longer than a normal corpse for signet of capture purposes though.
i think boss coprse last until you zone...the others are between 45secs and 1 min. Although if a necro uses it i think it disappears quicker...and not sure if you kill a bunch really quickly that they all stick around.....
