Blue card and your weapon registration will remain in their computer system until you change the status by going to them and doing so.
If you're a local and you sell me your "carded" handgun, "we" go down and register the weapon in my name and I get a new blue card w/ firearm info and they keep your old card. Then WE are both in their system, but I am now the "registered" owner. (Actually "We" do not have to go down together. In the past, when I've purchased your weapon, I take your blue card, a bill of sale you write me and the handgun and register it. They keep your old blue card and issue me a new one. Only once have I purchased one where the seller not have a blue card, or so he said, they ran the weapon #, came back and gave me my new card.)
If you do NOT live in NV, and sold me your handgun whilst in Clark Co., we'd have to have a licensed FFL involved and HE would give me my new blue card after the legally processed transaction occurs.
If you sell your "registered in Clark Co." weapon to someone else in your home state, Metro has no way of knowing that and you and the weapon will remain married in their system. I do not know if you could then go to Metro and "Un"register it, but I guess you could try it you deemed it necessary.
They do not expire in any other way. As one Cop told me, "Once you're in the system, you're in the system for life." A minor inconvenience and it does allow them to take "unregistered" weapons off the streets whenever they interact with someone breaking that law and gives Metro another reason for your ride downtown and processing. Probably not one of the sights you want to see when visiting Sin City.