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I'm all for the timer in player-versus-player battles. While the specific numbers might vary, overall, I think having such a timer as an option in the game is a good thing. Especially, as Hell mentioned, for tournaments. In a regular season, it could be made optional.
Now, we need to formulate the task. Let's continue. Regarding Sir Davos's "stepping away for a bit" and "strongly opposed" stance.
There should definitely be a confirmation button to start the battle – similar to the "end turn" button. One button at the beginning of the battle for each player. The timer doesn't start ticking until both players have pressed it. Only after that does the battle begin, the timer activates, and neither player can leave until the battle is over. And there's no problem (except for extreme slowness, which we're already trying to address).
There's one very important detail: FIRST, the player who was attacked confirms the start of the battle. Only then does the button activate for the attacker.
This is necessary for situations where one player goes for a smoke break while the opponent is taking their turn, gets attacked, and while they're gone, the first player also decides to step away.