After you get the reward, seven other characters have to get it before you can get it again. From my tests it doesn't appear to make any difference when a character possesses the balloon without getting the reward, nor does it seem to matter how many steps are taken or how much time has gone by since your last reward.
It therefore takes 8 characters to complete a balloon reward "circle", meaning that only after the eight character has gotten the reward can the first one can after it again.
BTW, by passing the balloon around in a circle for about 15 minutes, I was able to rake in a total of 29 gold nuggets, 9 wonderflasks and 8 treasures.
Sandy