We are in testing right now to find a happy place with regards to match length so I don't have a "they will be X min long" to share at this time. I will have more details in my blog post (that I mentioned in a previous post in this thread) that will shed some light on match length, among other topics.
Since it's a deathmatch scenario, it might be nice to have it to where each kill, removes some chunk of time from the clock - the greater the disparity in kills, the more time that is removed. That would make facerolls end quicker, close matches last longer.
Example: Each Arena is slated for 15min. Every death removes 5 seconds from that timer.
1-5 kills = -5 seconds ea.
6-12 kills = -10 seconds ea.
13+ = -15 seconds per kill from the clock
Similar to how a one sided WZ ends quicker with less or no objectives being held.