I think the best solution might be to have a character get forced into combat along with any of their summons. That's the only thing that would stop the exploit. (Assuming Larian wants to stop the exploit.)
You can't really put a duration on the totems in combat that isn't turn based, because then your totems would expire if another player had a really long term (e.g. multiplayer).
The exploit is made possible because of the hybrid nature of gameplay, which allows some people to be on a turn-based "time" and others to be on a real-time "time".
