Got the SSR weapon from my first platinum chest in Ultimate. I'd estimate I've done around 9 runs so far. If the platinum chest drop rate was something like 1/10, which would mean about 1 per 450 AP or once per 1.5 days, that'd be a definite improvement over Expert and you could expect ~5 platinum chests per event. That would make fully limit breaking either the SSR weapon or the SSR eidolon each event potentially attainable without elixirs.