Gladiator's Rucksack

Octopuss

Is there any official confirmation how often can you open them and get that special thing, and whether it's per account or per character?

Dragonnord

One once every 20 hours or so, and only one per account.
 

VaranisArano

I can only get the proof once a 24 hour period on the same character (haven't tried other characters). If I open more than one, I didn't get the proof after the first, though I don't know if there's a chance to get one like there is for the geodes from rewards of the Worthy.