There’s no meaningful viva prep that needs to happen before submission. Given the kinds of tasks involved – reading your whole thesis carefully, making notes, having some kind of meaningful practice with questions, and so on – an estimate of around 20 to 30 hours of work seems reasonable. But rather than simply block that much time out in your diary, I think it’s better to ask more useful questions about the post-submission period:
- How long might you have between submission and the viva?
- How busy is your life, or how much of your time is already accounted for?
- What do you think you need to do in order to feel happy about your viva?
- How long do you think that might take? (and how certain are you of that estimate?)
- Given the answers to the previous questions, how much time can you regularly commit to your preparation?
And finally given the answers to all of these questions, when do you likely need to start preparing for the viva?
Twenty to thirty hours could be right, but it depends on many factors. Pay attention to your situation and how you feel. Make a little plan and then take the time you need.