Is there a formal rule for this? If a PC or NPC runs from a battle and someone gives chase, they can both effectively run at 4 times their normal speed. How do you decide who give out first, trips, runs out of breath, etc?
You can run for a number of rounds equal to your Con score, after which time you must make a Con check (DC 10). Every round thereafter, the DC increases by 1.
Alternately, you can simply make opposed Con checks to see who's going to give out first.