The core hardware of a premium nuclear bunker air scrubber (like those from DEFCON Bunkers or [American Safe Room]) lasts 5 to 10 years. However, the actual CBRN filters (gas and particulate) require replacement every 6 to 12 months when operating, while the primary HEPA filter can last for years in dry, non-contaminated environments.

Bunker air filtration : r/preppers https://share.google/1ocXyhrcvc9nSWYoc

They only have so many replacement filters, if you keep pumping carbon monoxide in (heavier than air, so it will sink and be harder to pump out), the life support systems will break.

Unless they have a biodiverse ecosystem and infinite energy, they'll eventually need some input from outside of their bunkers.