Mine lose a fair mount of air if they sit for the week. I check my pressures every day before my first ride and wouldn't even think of wheeling it out after a week and riding it without checking them.
Interesting note about the molecule size between nitrogen and regular air, Tom. I didn't know that. I always just thought nitrogen was preferable due to less water/moisture and, in the case of aviation, keeping a more stable pressure with altitude differences.
We always used nitrogen in Army helicopter tires, but strangely enough never really regularly checked tire pressures with a guage and just noticed when they "looked a little flat." I think it was called for during a weekly inspection, but we never did it.
Even though I check my KLR tires religiously, I'm really bad about not checking our car tires. I can honestly say I'll go 4-5 months without paying any attention to them at all and that's probably not good.