From what I understand, the lifespan of PM depends mostly on two things:  the quality of the materials you use, and the care you give it.

Using non-acid materials is a tremendous start.  The residual acids in some materials (like newspaper) can break down the basic material over time.  I hear there are products that will remove acids from paper, but if you're thinking of going down that road, why not just use acid-free materials from the beginning and be sure?

Highly acidic materials in the atmosphere can also damage PM, but other than sealing and not having them downwind from a leaking acid-producing manufacturing site, there's probably not too much you can do about that.

Care:  I would say that moisture is PM's biggest enemy, followed by sunlight.  Exposing it to high moisture (even when sealed) like leaving a PM gnome out in the weather all year long may last for years, but I doubt that it will last forever.  Abuse will also take its toll, chipping and damaging the surface can allow moisture to enter. 

The ultra-violet part of the sunlight spectrum is pretty destructive on many materials: plastics, paints, sealers, roofs, fabrics, etc.  Keep your pieces out of direct sunlight.  Of course, if you're having guests over and want to show off your pieces in sunlight, fine -- just move them out of it afterwards.

There are still pieces of PM in good shape from fifty years ago, and there are even pieces from 150 years ago.  But I'll bet none of them were left in damp basements and garages, or sitting on a sunny windowsill year after year.

Just common sense will solve a lot of problems.

Have you posted your heads in the gallery here?  I would love to see them, and I'm sure others will, too.  You know, so we can say, "We saw some of her first pieces and knew her when...", after you are famous for your PM sculptures!

Sue