As far as I can tell, units defeated in a battle are unconscious for one minute. Then they are dead for two more minutes and disappear after that. When a saved game with unconscious units is loaded, these units remain unconscious forever (okay, maybe not forever, I waited only 10 minutes).
Steps to reproduce:
- Use any map and make sure there are some unconscious units (empire units or player's minions)
- Save the game and reload
- Observe unconscious units - they will not die
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