First things first mini run a timing chain for the cam shaft, no timing belts?
The super charger/aux belt I change mine every 10k or every year but I am running a smaller pulley on the sc.
I am not shore what mini recommend, but it a cheep item and it not worth the risk, some people have had 30k from them some have has 100 miles
My advice is if you can change it your self, change it every year or 10k to 15k or every year at your service. I have seen the mess they leave behind when they brake, and it can be a big bill, belts are around £15 to £20 quid mark if you go for oem part or about £12 from the states for a Goodyear or gates one
The belts take 15-20 min to change
Hope this helps you out