Well, during the 1st Phase it was moderate - like 1 card per 5-10 silver cases (but almost never in the gold ones, and i once had to open like 20-25 cases before i got her at the last day of the Phase 1)
Now, in the 2nd phase i'm getting Eri Aoyama almost in every 2nd case i open - today i even got 5 of them in a row by opening some of my own cases and using keys on Throw's cases that had 20-40 minutes left until expiration.