I think careful definition here is necessary what a "print life" is.
In principle, under "normal" circumstances (i.e. run in present-day normal movie theaters with so-so trained projectionists) a print often has scratches and dirt just after a few weeks. This is due to bad handling of the print. One could argue that after that time the print life cycle is over.
If you're unlucky and the print falls into the hands of an incompetent projectionist, the print life may be over (quality wise) even before the first screening (I've seen it happen, with my own print no less).
What accountants usually mean by "print life" is the economical print life. Most feature films stay in the cinema less than 7 weeks, so even if the print is still in pristine condition, what bookkeeping is concerned the "commercial life" of the print is over, so print value in the books is zero and most prints are trashed/recycled (because they cause storage cost).
If you don't care about the print condition too much, a print can live 40 years and upwards when run in theaters. There are quite some archive prints which are original release prints from the 70s! But usually they are quite beat up.
If you take really good care of a print, a print can have a very long life (even if you're VERY critical of print quality). This is one of my pet peeves. I've worked as a projectionist myself and I've been collecting 35mm projection equipment for some years, so I know where the bad things happen. Most projectionists let the leader touch the floor (where it picks up dirt, which migrates into the film), don't clean their work rooms and their projectors, thread sloppily with wrong loop sizes etc, don*t get their projectors setup/fixed properly etc... if all of this was done right, you could screen a film for years and years without any degradation in quality visible. Especially the Kinoton FP30 series is very gentle on film. I've run old brittle, shrunken prints from the 50s on these and never had any film breaks nor scratches!
As an example for how long a print can stay in perfect condition, there are quite some 70mm prints from the 60s which look like new (ignoring color fading). Those 70mm prints were and still are very expensive, so every projectionist is super careful not to damage it (and also no unexperienced projectionists are allowed to run it).
So the moral of the story is: when you own a print that's dear to your heart and you let someone screen it... go there with it, find out about their equipment etc, talk with the projectionist, possibly clean and adjust their equipment (if possible) etc, and if possible (i.e. in case they let you do it and you do know how to): build and thread it yourself.
Edited by Marc Roessler, 20 July 2013 - 03:59 PM.