How long does it take to hear back from EY final interview?

It means nothing in terms of the outcome, don't worry. Most likely case is that the person who interviewed you hasn't yet written up the feedback and sent it to recruitment, or they have done and recruitment haven't got around to calling you yet.

I had my interview on 4/12 and was told I'd probably hear back the following week. The following week I got an email saying they still have more people to interview and that I may be waiting a few more weeks before receiving my outcome. Then my portal changed to 'offer' on 16/12 and I received an email that day asking if I was free for a phonecall the following day, in which they offered me the job.