I would say that the normal fatigue rules would work well, being Con modifier in rounds, then start making Con checks... But I guess I can see why it wasn't necessarily implemented. Each character would have a different round to start checking, then the DM would have to keep track of what round what character started making Con checks, and all this could just add complications and extra dice rolling to the game. Although, I have noticed the more dice rolls involved, the more "realistic" a combat "seems" to become, but its still not going to mirror reality, and still ends up being random (that having to do little with fatigue, just off topic I suppose)
Admittedly, combat rarely takes more than 20 rounds, so I guess it is not all too important. (but I still think it is sensible).