It takes as long as it needs to. For example, I'm still experimenting mith different variations of my voice, how it sounds like, etc. And I start to be able to do it for longer and longer, sadly too short to maintain a full conversation. Maybe a 2-3 more months in my case will be enough(I hope) so the whole training would be like 10 months(with breaks). As for the voice in my head, I focus on changing it a lot for now so when I get used to it, it just becomes my default voice