I've been testing leash behavior in the Beta and found some interesting results.
Mobs chase you for different lengths of time depending on level.*
1-29: ~11 sec
30-39: ~12 sec
40-44: ~13 sec
45-49: ~14 sec
50-60: ~15 sec
These are only a guideline however. I have observed that a level 40 Bloodfen Lashtail will chase you for 13 sec while a level 40 Bloodfen Razormaw will chase you for only 12 sec. The reason for this is the Lashtails can be 40/41 and thus belong to the "early 40s" bracket while the Razormaws can be 39/40 and belong to the "30s" bracket.
Similarly, a 45 Highborne Apparition (45/46) will chase you for 14 sec while a 45 Blisterpaw Hyena (44/45) will chase you for only 13 sec. So the above values are correct with respect to the lowest level the mob can spawn as.
Many people think mobs leash based on distance. This is how it works on private servers but was never the Blizzlike behavior. I have demonstrated it's not based on distance using a variety of tests: walking slowly, serpentining instead of walking in a straight line, etc.
However, that's not to say distance is not a factor at all. A mob will chase you forever so long as you remain within a small area near its spawn point (or possibly near where it aggro'd you / where it hit you last).
Based on my testing, this range is about 10-15 yards, which is too small to execute maneuvers like the "kite-->bandage without resetting" trick which Monkeynews used to solo Hogger as a warrior in his Elwynn speedrun video.
Extending the leash
The leash timer starts over when the player takes a hostile action against the mob, enabling you to kite Gamon to Silithus if you Curse of Weakness him every 10 seconds the whole way there (which I did in vanilla).
However, yesterday I discovered it is possible to extend the leash with no action taken by the player (indeed, I was doing it while mounted): if you kite the mob for several seconds, then the next time the mob swings the leash timer will be reset.
This behavior ONLY occurs on the first melee hit after a period of kiting. It does not work with spells or ranged attacks.
Here is an example:
If I aggro a gray mob and kite it without being hit, it will reset after 11 sec.
If I aggro a gray mob and run away while it beats on me the entire time, it will reset after 11 sec (11 sec after the first melee hit to be precise).
If I aggro a gray mob and kite for 10 sec then get hit once, the leash has been extended. I can then resume kiting for another 10 sec, rinse/repeat.
I discovered this behavior yesterday while waiting in Arathi Basin queues. I'll drop a link here:
Sharptalon is level 31 yet leashes as a 1-29. I believe this is because he is in a lower level area.
Starving Blisterpaws (41/42) leash as if they were 1-29. This is a full 2 seconds shorter than they should, or than any other mob around them of the same level. For this, I have no explanation. Maybe they were created as a copy of a lower level hyena and somebody forgot to update their leash behavior?
Great stuff. I will add this into the next Beta vid man.