What is the time for sufficient decay of plutonium to render an implosion type fission bomb ineffective Plutonium decays leaving helium.  In a mass of metallic plutonium, some of the metal will decay resulting in helium. This makes the plutonium spongy after time.
Presumably this sponginess would impact the performance of a implosion type fission bomb, by compromising the structure of the plutonium and also making it less rigid.  
What is the time frame for plutonium to decay to the point where the resultant metal would be incompatible with the structure required for effective bomb operation?  In other words, when does the bomb get "too old" to effectively operate?
Additionally, the half life of Pu 238 is about 87.7 years, which factors into the question, but the question is really about the nature of the decay and it's impact on the structural properties upon which are assumed in the design of a particular type of implosion fission bomb.
NB, I realize that small amounts of tritium are used to dope the reaction, and the half life of tritium is rather short (like 135 days) so for the purpose of this question, please ignore other factors such as tritium, shelf life of conventional explosives, etc.
 A: The exact answer would likely be classified because it depends on fine details of the warhead assembly. However, there is some info in the open literature. Two nice papers are Wolfer (2000) Radiation Effects in Plutonium and Hecker & Marz (2000) Aging of Plutonium and Its Alloys.
Plutonium is a very annoying metal to work with due to its complicated phase diagram, with different crystal phases having noticably different densities, the heat emissions from the metal driving spontaneous transitions (which, since they have different volumes can lead to cracking), high reactivity, plus of course the radioactivity and toxicity. Apparently it is commonly alloyed with gallium or other elements to make it more manageable and keep it in the $\delta-$phase.
Plutonium-239 decays to uranium and helium, and the helium will tend to remain in the lattice. It might accumulate into bubbles at lower temperatures. However, there is also an effect of the recoil and dislocations induced by the decay: every plutonium atom will be displaced, on average, once every 10 years. Dislocations build up and coalesce into voids, producing a void swelling of the metal. This is a bigger problem than the helium.
The time until stabilized $\delta-$plutonium swells by 1% looks like 40+ years in fig 9. of (Wolfer 2000), and might then keep on increasing by 1% per ten years. But as he points out, this may take longer depending on metallurgic treatments and shape. Worse, radiation damage can also cause local shifts to $\alpha-$plutonium that has 20% smaller volume. All of this is also very temperature dependent. (Hecker & Marz 2000) say:

However, general observations of self-irradiation damage show no major
  macroscopic changes for at least  40 years—in other words, plutonium
  does not “crumble”. There also appear to be  no gross microstructural
  changes, such as phase changes or segregation. Early studies found a
  slight volume expansion in  δ-phase alloys, on the order of 0.3 percent in  10 years.

As a final amusing note, they point out:

After 50 years of storage, weapons-grade plutonium will have grown in
  the following amounts of transmutation products: approximately 2000
  atomic parts per million (ppm) helium, 3700 ppm americium, 1700 ppm
  uranium, and 300 ppm neptunium. After that length of time, a piece
  weighing 1 kilogram will contain nearly two-tenths of a liter of
  helium measured at standard conditions. Said differently, after 50
  years of decay, the accumulated helium in plutonium would generate a
  pressure of 3 atmospheres in an equivalent empty volume!

Would that make the pit unusable? I have no clue. But I think it is starting to run up against its "Best use before" date.
A: As commented by PM 2Ring, the spontaneous decay of Pu-239 releases energetic alpha particles within the solid chunk of Pu, eventually leaving an atom of U-235 in its place- so the Pu chunk does not become "porous" in this way.
Those alphas may then strike other atoms (including Pu of course) on their way out and can then trigger other nuclear reactions which may release different sorts of decay products like neutrons. 
Neutron flux in particular is a bad thing, as it compromises the metallurgical integrity of bomb materials either by triggering transmutation reactions or by a process called neutron embrittlement. 
This means that (in general terms), the half-life of the plutonium itself is only one factor in determining the "useful shelf-life" of a fission or fusion weapon. The other important factor is the accumulation of radiation damage of various types within the bomb structure, which might make it malfunction. 
