I have had it parked for almost three weeks with the tires on. How long before that creates a problem?

 

I've heard that you should move the tires every 1.5 months. Biggest factor I understand is cold. They aren't as plyable when there in freezing temps. I've heard that you can slightly overinflate and it helps. Don't take my word for it. I'd go research it a little more

 

I stored two vehicles in my garage for 3 winters (November - April) when living in Toronto (Canada). Temperature were anywhere between 0 and -20 celsius).

Neither car was moved (Mustang convertible and Firebird Formula) and just stood on the tyres for close to 5 months straight. The only thing I did was start the engines around once a month to keep the batteries charged.

Never had a problem with tyres or anything else for that matter.