Viva preparation doesn’t have to take a long time. It isn’t a huge amount of work, not compared to the scope and scale of a PhD.
It doesn’t take long, generally, to read a thesis, make some notes, capture thoughts and rehearse for the viva. A few weeks can be enough to space the work out. A few weeks of reflection and preparation.
A few weeks to remind yourself of what you’ve done, how you did it and why you’re capable of succeeding in the viva.