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Currently there's a hard limit on how long a spar can last, and it's fairly short. While it's generally sufficient for sparring, it's rather awkward for testing and novice training purposes, often requiring two or three separate Arena entries just to finish up your work.
I also understand that the time limits are necessary as other people could be waiting for their turn. But if nobody is waiting, there's no reason for there to be a time limit in the Arena.
So I propose an ARENA QUEUE mechanic. Basically, if you wish to use the Arena and it's already in use, do ARENA QUEUE CHALLENGE (person). If (person) accepts your challenge, a five-minute timer is placed on the combatants in the Arena. At the end of this timer, you and your party will enter the Arena. Otherwise, the Arena will be in use without a timer at all. If you or a member of your queued party leave the staging grounds during the timer, you are removed from the queue (as is the timer if you were the only queued party).
Multiple parties can ARENA QUEUE, and they will be allowed into the Arena in the order in which they queued. If two or more parties are queued, the first party in automatically has a ten-minute time limit which is not modified by more people joining the queue.