Viva prep is not the kind of work that has to be compacted into a short period of time. It can be done that way, but is probably far better to give yourself space and time to think.
Explore your situation and consider: when will you do the work? How will you do it? Where will you do it? How often will you sit down to read your thesis and when will you begin?
You can pace yourself. An hour a day. Five days a week. Four weeks. That could be enough to be ready.
Of course, you might want to take an afternoon off to do a big concentrated burst of work; you might need a few hours in a row to take advantage of a mock viva opportunity.
Pace yourself in a way that works for you. Find the space in your schedule when you can do the work and not stress yourself. Do the work in a way that doesn’t add too much to an already busy schedule.