The tritium *booster* can last 8 years or more and still be useful. The shell/case lasts even longer. The atomic *trigger*, however, has a shelf life of about 90 days.
Besides that 90 day time limit for the atomic trigger, you've got to remember that the radiation inside the bomb does a number on the associated electronics and conventional explosives.
One of the things that the Russians had to steal from us was the chemical recipe for our high explosives (RDX??) that we use in nukes because their own conventional explosives were...hmmm...how to say...having difficulties.
Also, keep in mind that Uranium and Plutonium are metals. Brittle, brittle, brittle metals, in fact. Oh, and they rust rather easily, to say the least.
So anyone with an old backpack nuke from the former Soviet Union (deceased for more than a decade now) essentially has a corroded, fragmented, decayed "dirty bomb" instead of a nuclear weapon.
If you want to have a greater than 50% chance of your atomic bomb actually chain reacting, then you need to perform laboratory-clean maintenance about every 3 months (replace the atomic trigger, test the electrical wires, replace the electrical components, test the humidity, replace the conventional explosives, etc.). Then about every 8 years you will want to replace the booster material (usually tritium) and every ten years or so you'll probably want to replace the core and shell/case.
And these aren't things that just any auto mechanic can perform. Move the core too close to the case/shell at any point during such maintenance and you'll be short one atomic maintenance crew. Create a spark or power surge while changing out the electronics and the high explosives may reach out and say hello.
Keep in mind that the entire might of NAZI Germany failed to get one of these "right" during 8 years of trying. It ain't easy. Most of what you read in the press is a fantastic oversimplification of the technical hurdles involved with these monsters.