Bornakk just posted an important announcement for all arena participants:
There is now a time limit of 45 minutes for all arenas. If neither team has achieved a victory, both teams will lose 16 arena rating points.
This is posted in the general discussion forums, and we can safely assume that this change is active on live realms.
Bornakk goes on further to say that matches that go on for 45 minutes are currently showing up as a draw and not displaying a loss of arena points. However, the point loss still happens. This means that you can apparently see it when you look at your honor page and you'll notice that your rating has gone down by 16 points
We've covered some very long arena matches in the past. A 13 hour match between a Druid/Warrior and Warlock/Paladin, won by the Warlock/Paladin, and a 10 hour arena match (the classes were not reported on).