Probably like 5 minutes... I accidentally shot one of them and when it updated I was like "Wait, what?!" I really liked the quest as far as Pelagius and the lore behind Solitude (Hip Bone, hehe). Idk, it reminded me when I read about him and Potema in Oblivion.
I shot the people on the other side first thing, but it didn't work... so i thought I was supposed to shoot my atronach to be stronger that the other one... ended up giving up and shooting random things and ended up shooting one of the guys and it worked...
but after I did all 3 things, I honestly could not find out what to do... the guy you talked to before didn't show up for at least 20 minutes and it really pissed me off... Bethesda you need to make your ps3 games waaaaay better...
Ironically, I went for the armored guys across the pit first but apparently missed. Then spent 10 minutes with the atronachs before trying the armored guys again. Ah well.
In my first playthrough I solved it in a instant because I was happy to have the wabblejack and was randomly firing it at things and didnt even noticed I hit the guy....I thought It glitched or something and I automaticly passed the test....
but on my second playthrough Oh boy I spent a good 10-15 mins not knowing what I was doing because I didnt know I was suppose to hit the guys.
It took me a good five minutes. I kept shooting one atronauch hoping that it would change to the other atronauch's weak element haha. Everytime I shot one, the other would change too, so I thought I just wasn't shooting fast enough.
About 10-15 minutes (spent at the arena). I tried all the different combinations of combatants (all three of them) and cycled through them repeatedly hoping for a different reaction. Then I chose the one that seemed most effective (fire vs frost I think) and waited for a few minutes. Got frustrated, left to do another section and came back to find that nothing had changed. Eventually after looking around the arena for any other ways to influence the battle I noticed the Dwarven armored spectators and tried shooting them.
It took me awhile, because I figured it was some kind of elemental weakness thing, but I didn't have as much trouble with that one as I did with the shrinking guy. I never figured that one out and must've only done the right thing by chance (or Sheogorath just got bored).