At one point in the book after the resupply, when the Hermes is on the way back to Mars, Beck is giving a status report and says, w.r.t. life support:
Limping. We've been in space way longer than it was designed to handle. There are a bunch of filters that would normally be replaced each mission. I found a way to clean them with a chemical bath I made in the lab, but it eats away at the filters themselves.
So, they're consumable filters that the crew can reach (as well as the 'tween mission ship maintainers) and the actual lifespan (and presumably the expected lifespan as well) is less than two Mars round-trips. And, you know, life support!
Why didn't NASA send some up on the resupply ship? NASA oversight? (Or author oversight?)