Transition is like a buffet. Just like a buffet, there are many different things you can take as a part of your transition and many things you can leave behind. Just like a buffet, if you decide you want something that you left behind, you can go back and get it later. And just like a buffet, the things people put on their plates for transition are different for everyone.
So I think the answer to the question in the topic (how long SHOULD one work on voice) is: as long as they feel they need to.
As shown by the answers to the question posed in the OP's text (how long DID people work on voice): the answer varies widely from person to person depending on what they felt they wanted/needed. For some the answer is: not at all. For others the answer is: lots of long hours. For me the answer is: not applicable.