It’s definitely possible to over-invest your time in viva prep.
I started six weeks before my viva. I probably spent close to 200 hours getting ready.
That’s a LOT more than is needed.
It’s also possible to do too little or start so late that you need to rush to squeeze everything in. There’s a worry I’ve seen expressed by many candidates over the last decade about finding the right time to start.
Two weeks before? Three? A month? When?!
If you submit your thesis and find yourself wondering whether or not to do something, then do it. Just do something and release the tension and the wondering. Viva prep is cumulative: it all adds up no matter when you start or do it.
Most candidates don’t need to start preparing until a month before the viva.
Most candidates would probably benefit from having at least two weeks to spread the work out sensibly.
But if you’re wondering if now is the right time then just start.
Just read something. Just write something. Just talk. Just start and do something and know that you’re on your way to being ready for your viva.