I tihnk this topic is very much like tax
Id depends how far you like to push the boundaries
You must occupy the property as you principal place of residence within 12months for a reasonable period of time.
The problem or advantage is the defn of "reasonable"
If you are conservative then it might be 12months
If not it might be an hour, a week, a month.
What I think is important is to show you did actually occupy the residence for a reasonable period (ie be able to substantiate this)
For example what happens if I did buy a property to REALLY be my PPOR I moved in, changed all my mailing details, drivers licence, electoral role etc moved in, connected a few services (say water/gas/elec) and then after a month I found i was already behind in rent or a family member got sick and I had to move out of the PPOR and make it an IP because I couldnt afford it / had to look after the sick family member
In this case surely you'd have to define this as a reasonable period of time.
Its amazing how many ppl this has happened to
Does that make sense ?