Well, look at it this way: if he was still around when Viola collapsed, then that means that the pod only had to operate for two years on its own (facility two was shut down two years after the first facility went dark).
Considering that the gel fluid is highly oxygenated, and that being in them is close to a state of hibernation, I think it's not TOO much of a stretch (it's a stretch, but hey, it's supersoldiers XD) to just say that, because hibernation requires so much less oxygen, that it would last a lot longer than if someone was awake inside.
Maybe Cass wakes up when the oxygen levels are low?