For a reduced-pressure, easy-going period of viva preparation you might need a month.
Between thirty minutes and an hour of work each day will cover you.
Read your thesis at least once. Make notes and additions to help your thesis be even more useful in the viva. Write a summary or two. Check references and useful literature. Simple, deliberate, scholarly work that builds on the thousands of hours you’ve invested already.
Rehearsing for the viva itself – with a mock or having conversations with friends – might take a little longer than an hour, but for the most part you need only a month of short work periods.
Sketch a plan. Gather your supplies. Build your confidence with thirty days of small differences, small improvements on the great place you’ve already arrived at.