The silver coffers are entirely random, so it's difficult to say just how many runs you would need to do in order to get on i235 weapon. I've been in three times--only two times actually counted for my weapon/armor--and they're both at +17.

The lower you go, floor-wise, I think the more likely the silver chests are to just blow up in your face. That happened A LOT to my FC and I when we got into floors 41-50. Only one actually gave us any +1.