While potentially infinite it is ultimately limited by the number of enemies present (and how those enemies are positioned relative to one another). I had a player take out quite a few minions with it as it was a perfect storm of lots of minions on a battlement spaced out relatively evenly and the player getting good roles. However, in most fights, they may be able to hit all the enemies in the fight (which is quite good, but requires quite a bit of luck), but unless it's a ton of minions, it's going to be a nice ammount of damage, but hardly deadly. The "no same target twice" is what keeps it in check.
Assuming it isn't too hard to hit (say about 70%), you end up with a 35% chance of getting a second shot off, which has comparible odds. The chances of "going infinite" are slim ... played long enough, it will come up once in a while where a long enough chain will happen but not so often to make encounter or daily powers useless compared to the at-will.