Consumer Reports did a quick test and found that N2 leaks out of tires only about 80% as fast as air. What that means is that the O2 in the air leaks out faster than the N2 in the air. That means that the air in your tires gets "nitrogenier" over time. Water molecules are also smaller than N2 molecules. So, the air in your tires get dryer over time, too.

I check my tires every two or three weeks, and before and after a day on a road trip. By doing this in my driveway before sunrise, I can adjust the pressures of the two tires on an axle to be exactly the same. If one is lower the next time I check them, I go looking for an embedded nail or screw.