how long until tires flatspot?

I have tires on a car that it’s last inspection sticker was 1985 when it was parked in a garage, I picked up the car in 2000, today it still has the same bias ply tires on it.

So more than 180 months.

But seriously make sure the tires stay inflated, if you are really paranoid move the car one foot or so every couple of months and it should be fine.