Here's how I see this nitrogen debacle.
Assume you fill up with regular old air, 80% nitrogen. Pretend the rest, regardless of composition, leaks out in a week.
So, in a week, your tire is at 80% capacity with pure nitrogen. You now fill up with regular old air. Now 80% of that 20% is nitrogen, and the rest leaks out.
Now, two weeks later, your tire is at 96% capacity with pure nitrogen. You fill up again with regular air. Now 80% of that 4% is nitrogen, and the rest leaks out.
This is why I can't get my head around nitrogens claimed benefit of less leakage. It seems to me that if that were true, tires would naturally become pure nitrogen over time, or effectively so.
Of course this is an oversimplification, but it's an illustration of the disconnect that exists here.