Rotating tyres is not something I used to do. Apart from when the Subaru dealer used to rotate them every service.
My reasoning for not rotating is that when I need tyres it will be two at a time and therefore easier to budget in the shorter term.
I have rotated the winter tyres on the Volvo mainly due to the fact they have been on there all year, before anybody says anything about that the car was hardly used during the summer and to be honest this year has been very wet and not particularly warm.
The front winters had a couple of mm extra wear than the back so I have rotated them to even up the wear.
My wife's car is very hard on front tyres, heavy Diesel engine and the wife does a lot of urban use as a Care Worker so lots of three point turns!
I therefore don't really have a specific time limit.
But discount deals for buying four tyres at a time a relatively rare in the UK, I think if they were as common as they seem to be in the US (going by comments on bitog) then I suspect I would be much more attentive to this kind of thing.
The only really good four tyre deal is Costco, and that is a subsiduary of an American Company I believe.