This one is, I think, a little more graceful.
Characters may fight continuously for a number of minutes equal to their current hp total. Since I use C&T, this means hpx4 rounds (each round is 15 seconds). This would necessitate the number be recorded next to hp and ticked down each round.
When the number of minutes fought exceeds the number of remaining hps, the character becomes fatigued and receives a -2 penalty to all rolls, AC, saves, etc. save for damage rolls. This condition remains in place even once the fight is over. By taking a 15 minute rest after the fight, this penalty can be reduced to -1. Another 15 minute rest will reduce it back to 0 (though the current hp will still be used to calculate another burst of fatigue in the next combat).
Simpler and, I think, more effective.