Decades--as long as it is a) kept in a shock, temperature, and atmosphere controlled environment, a pit can last for many decades.
The primary barrier used to keep modern plutonium pits from air and moisture (corrosion) is to electroplate them with gold--which is a good idea anyways since plutonium can spontaneously ignite in air if plutonium powder is allowed to accumulate. Keeping oxygen away from plutonium metal is always a good idea...
Keeping the pits in a shock resistant equipment case--placing the pit in a polyethylene ziploc storage bag stuck in a foam lined storage case ought to keep the pit safe for decades.
The primary part in a nuclear weapon that must be periodically refurbished are parts that depond and contain tritium--tritium which is neutron rich has a half-life of 12.3 years, and decays into helium-3 which is neutron poor. As such, helium-3 contamination in a tritium booster located at the center of a modern boosted fission weapon can render its yield unreliable...in some cases it may even fail to reach a nominal yield (if too much helium-3 accumulates then the 'booster' becomes a neutron sink instead of a supplementary source...
If I were in charge of keeping a 'healthy' nuclear weapons stockpile, I would want to service and rotate out those old initiators every five years or so...
I was thinking a bit grander than maintaining weapon stockpiles
I've just been doing some "Back of enverlope" stuff for an Orion starship (With refrence to a short SF story)
My hypothetical Starship will have a transit of about 1000 years. My guess is that Pu239 will *not* keep for 1000 years because radioactive decay would affect the purity too much (though I may be wrong)
(My guess is also that HEU probabally would *keep* but the quantities required would require the mining and processing of 10's of millions of tons of Urainium)
What this means of course is that the deceleartion pulse units will likly need to be manufactured to order rather than loaded at launch. Which in turn means that the crew area will need a substantial industrial capability!
(It has also occured to me as a result of this exercise that, whilst not wanting to preclude the possibilies of some sort of *magic* warp drive, the solution to the "Fermi Paradox" is that though starships are technically possible, they are Sooooo expensive in resourses that no civilisation can afford to build one!)
I suspect that a viable interstellar craft will probably be the size of a space colony--miles across and carrying thousands of people. Only a colony that possesses a viable fraction of human experience and expertise will be successful, and even then only if they're lucky.
I suspect that more efficient means of interstellar propulsion will avail itself eventually--although what that technology will be is anyone's guess at this point.
Nuclear pulse propulsion is a possibility for large, "in system" craft--but it is really pushing the 'envelope' of the physics of the thing to suggest even a one way journey to Alpha Centauri, let alone a voyage to some of the likelier habitable candidates within 20 light-years.
In the novel by James P. Hogan "Voyage from Yesteryear," Hogan's spaceship Mayflower II is described as a rotating wheel about six miles across. Powered by thermonuclear fusion, it even possessed the ability to reprocess nuclear weapons---which does indicate a significant industrial capacity onboard.
In my "Scenario" the starship construction is an act of desperation taking the economic and material resources of the entire planet to achieve.
The enterprise runs into major dificulties because when they arrive at their destination (Earth in 1941) after a voyage of around 1000 years duration, the prospective colonists get a very nasty surprise!
Meanwhile, back in the real universe! ISTM that, unless a *cheat* is discovererd that doesnt require megatons of unobtainium or yottawatts of power, then we are going to be stuck with the rocket equation, one way or another.
This most likly means some sort of fusion drive, To my mind the best we can probabally do is a suitable contained a-neutronic reaction such that all the "products of combustion" can be directed magnetically to provide thrust.
This being the case I guess the best we will be able to achieve is a mission DeltaV of around 30,000KPS (IE Cruse at 15,000, everybody seems to forget that you have to slow down again!)
Starships are likely to be slow and therefore the "Payload" will have to be large. The quantities of fuel required are almost beyond comprehension. (Enough fusion fuel to meet the entire global energy requirement for thousands of years! )
Technically speaking, we are almost at the point where such a project might be do-able. However, the resources required!!
It is my suspicion that applications of the quantum vacuum may yet give us a way to accomplish interstellar propulsion.
There are some features of the concept of a blackhole which mimicks more esoteric processes--the event horizon of a black hole is very similar phenomelogically to the flow of gas through a deLevaal (convergent-divergent) nozzle in the sense that a disturbance downstream of the nozzle in the supersonic flow cannot flow back into the convergent section--i.e., pressure disturbances or sound originating in the supersonic section cannot back propogate into the convergent section of the nozzle, just as matter or radiation falling across the event horizon of a black hole cannot 'escape' back out. So in this sense, a de Levaal nozzle can be thought of as a sort of 'acoustical black hole,' or a mechanical analog of the space-time phenomena.
Interestingly enough, there are presently several high-energy particle accelerator projects in the world that have as their goals the study of black hole evaporation. It is thought that the Relativistic Heavy Ion Collider or RHIC (prounounced Rick) will be able to produce nanoblackholes by slamming gold nuclei into uranium nuclei, or maybe even two uranium nuclei together...with the purpose of studying the Hawking decay products with the hope of narrowing down the number of expected extradimensions and possibly their ultimate configurations...
However, it is my personal belief that if such an experiment proves successful in producing nano blackholes then it may be possible to eventually create one and force feed it enough matter matter so that when it does decay the energy deposited in an absorber may exceed the energy put into running the accelerators by enough of a factor to be a viable net energy producing scheme. In other words, create micoblack holes to feed matter to so that matter is converted, eventually by collision and annhilation, into energy in usable quantities. If we can do that in a flight weight package that achieves enough power generation, then it is concievable that we can propel spacecraft--even very large ones--by burning ordinary matter into energy. The energy, if great enough, could even directly heat a working fluid to very high temperatures--or it could be harnessed to generate electricity which can then be used for some other propulsion system...
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