time span of submission of thesis to viva (MRes)

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Hello,
Generally, there is a time span of six weeks between the submission of an MRes thesis and the viva.
Is there a reason why it is normally six weeks and not less time?

I submitted mine and the time between submission and viva was two weeks. To fit my viva on the same day as some other students having their viva (the university chose to do this).

thanks

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