Years. I know this may not be the answer you want to hear. But it's the truth. It takes years and years of hard work, studying, taking classes, practicing, taking more classes, and did I mention... taking classes. Most voiceover students take classes once or twice per month. But you should also budget lots of practice time and self-study.
It takes anywhere from 2 - 10 years to become a working professional voiceover talent who is consistently booking work and maintaining repeat clients. The time you invest is critical to your success, and will also be to blame for your failure if you try to move too fast.
I see far too many people building voiceover studios, creating voiceover 'demos' and preparing to market a business they haven't yet begun, all without ever having taken a single voiceover class!
Be ok with being a student. It's not a sign of weakness, but rather a mark of intelligence. Let humility be your ally. If you are reading an FAQ about voiceovers you are not ready to dive head (and wallet) first into this.