Viva prep is needed to help a candidate be ready for the viva. We could say that if you start your prep three weeks before your viva, then after ten days or so you’re halfway to being ready.
But when you think about it, you might do more work in the week immediately preceding your viva. So then being half-ready skews more to the days just before your viva, when you’re working more intently.
Or maybe it’s most useful to consider: viva prep is spread over several weeks at most, whereas the real work of the PhD takes several years.
Viva prep is a focussed period with one goal, making sure that you are ready for the viva – but by the time you start that prep, you are definitely more than halfway to your goal.