I had a luxury of time to prepare for my viva. I didn’t have a job or a family, so I treated my prep as a continuation of my 35-to-40 hours per week routine.
Most candidates won’t be in that position. You could have a full-time job, or a part-time job, or family obligations or 101 other things that I didn’t have to think about.
And that doesn’t need to be a problem. It just takes a little planning to make sure you don’t feel overwhelmed.
There are lots of things you could do to get ready (and there are lots of posts on this site on those sorts of topics!). Think about what will make you feel prepared. Explore how much time you might have, based on when you might submit and what your work pattern is like.
Break things down. How long would it take to read a chapter of your thesis? How long would it take to write a summary of a chapter? How could you spread it out over the weeks before your viva?
Even if you don’t have specific dates and times now, you can map out roughly how you might get it done. Keep that rough map. When your thesis is submitted you can start to make that sketch a reality.
Viva preparation doesn’t take hundreds and hundreds of hours. If you’re busy, or you’re going to be, sketch a plan today.