Nearly every person that I’ve spoken to about mock vivas had theirs about two weeks before the actual viva. For them it was a chance to explore their thesis, get questions about their work and see how they would feel responding in a viva-like situation. Most people want something like that from the mock.
A while back I spoke to someone who had their mock viva a month before they submitted their thesis. They wanted to see how well they were communicating, both through their thesis and through the answers they gave to questions. Their early mock viva gave them a chance for “pre-corrections”: based on questions and feedback they tried to improve their thesis as much as possible before submission. That didn’t mean that they weren’t expecting corrections later but they were using their mock to make their thesis the best it could be.
I’ve never interviewed anyone else who has had such an early mock viva. It might not be a terrible idea to do something like it though. Why not host a seminar or have a series of conversations to unpick how well you’re communicating your research? What could you do to improve?