Nukes are not that high-maintenence. A suitcase nuke would have to be VERY low-maintenence in order to not be useless under any conditions. What? Are you going to wait for a declaration of war to smuggle these in behind enemy lines? Hoping these are past their "best if used by" date is no protection.
It was my understanding that the nuke itself might have a long expiration date (though not indefinite), but the triggers are relatively volatile. You might expect to keep an unarmed suitcase in-country for 5 years or so before it would have to be serviced. However, the trigger would probably be sent/smuggled in only shortly before such a device would be used. If you didn't use the trigger within a year or so, it would degrade to the point that it would no longer work.
And Southack's post 42 has even more detail.