I'm not an accountant - but I understand the general principle...

I'm not an accountant - but I understand the general principle to be that the ATO will look to see if there is any commercial purpose served by the transaction other than to minimize tax. If the sole purpose is to achieve a favorable tax outcome, then the Commissioner has power under Part IVA of the tax act to disallow the benefit.

As such, I doubt that there is any "rule" about the number of days you have to wait. It would be assessed by the ATO on a case by case basis, with regard to the particular circumstances of the sale/repurchase.

As I said, though - I'm not an accountant. So would defer to expert knowledge.