I started my prep a month before but my prep time was restricted to evenings only. I found by the end of the month that I was going around in circles and not sure what more I could do so I took a couple of days break just prior to viva day. Re-reading my thesis was the most time consuming as I also noted errors, put post-its in to mark key places, added notes for clarity and generally refreshed my memory. Once done, I checked for any new literature then I worked on key Qs, writing down answers for later reading and talked through my answers. Talking through the answers really helps you to express the ideas verbally as this is what you will be doing in the viva. My children and the dog really had an education lol.