How long will the viva be? How long should it be? If it’s long – or short – is that bad?
There are norms – two to three hours is quite common – but you can’t know in advance. It could be less, it could be more. Many candidates, I think the majority, feel like their viva passes in the blink of an eye.
The length doesn’t indicate something good or bad. There’s no correlation between the length of the viva and the outcome.
A successful viva is not a function of how long it takes. The time isn’t as important as you are.