How long does a laser last?

We're buying a new confocal but I'm concerned about a life span of a
laser.  Some people have bad lucks with Kr/Ar lasers and UV Ar lasers,
and others including ourselves have been using visible-light Ar lasers
for thousands of hours.  I think we'll get a Kr/Ar laser, but we may have
to reserve extra money for replacements if its life span is shorter than
Ar lasers.

Any ideas?

Tohru Murakami
Department of Anatomy, Gunma University School of Medicine
Maebashi, Gunma, Japan