shutterbug

In theory a tire without puncture, good valve stem, well mounted should not leak any air in a short period of time (i.e. 6 months).  This is the case if you have a new car.  I remember when my car was new, I only checked the pressure every few months and the pressure is good, no loss.  Occationally, I get a few pounds less across all tires, but I guess that was because I haven't checked for too long, like a year.  Then I replaced my tires without putting in new valve stems.  Big mistake!  I had to put some air in every now and then...

So, make sure you get a new valve when you buy new tires!