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I would suggest a progressive system for adding time to a turn, depending on the game's duration. For example: 20 + 8 (+0.10) + 1 (+0.05) – this means that at turn 112, 8 minutes and 10 seconds will be added to the turn time, and 1 minute and 5 seconds will be added to the time for each battle; at turn 113, it will be 8 minutes and 20 seconds and 1 minute and 10 seconds, and so on. Of course, this is only if such a system is feasible.