They were notified on Jan 5th. They destroyed the stored passwords on the 9th and informed the customers on the 1st of February. It seems reasonable.
Why would they inform the end user though ? The end user doesn't even know what Mixpanel is and would be confused if they emailed them directly. Informing the clients (BMW, Samsung etc.) is the right thing to do which they did. I'm sure they had to do a postmortem and make sure a fix is in place before informing the clients and urging them to update the SDK.