18 days per parent. Yield fare is about 3.5 cents per mile. (so $20 from NY to BOS, a few hundred for Transcontinental) It's calculated differently for international. Usually worth it for non-peak travel outside the US. Not usually worth it for domestic. Annual fee is $50 flat rate, no matter how many people are listed. I think it's called a Travelnet fee or something.
Also FYI, if someone does go international on your pass benefits they have to pay the yield fare upfront but, the taxes and fees come out of YOUR paycheck. This can vary on destination from a couple hundred for London vs $10-20 for Canada. It for $13 for Dublin.