How often are enterprise software applications upgraded by the user? 
The question is not about the frequency of major version releases of
that software by vendors, rather how often are they applied by the
customer?
So for example if a company installs an application such as SAP,
running on top of Oracle, how often will new versions of the software
be applied?
Or another example, how often have microsoft issued a major new
windows release and what is the actual frequency with which companies
upgrade.  For instance MS issued Windows 95, 98, 2000, XP etc. however
not all companies took all upgrades, some skipped upgrades.
So with respect to enterprise applications such as CRM, ERP, SCM, what
is the average time between major upgrades?
A second related question is "what is the lifetime of an average
software installation"?

This is indeed true. The life time of a software version at the end
user's end is much longer than it is at the Vendor.

If we take the example of Microsoft only, MS has come up with not less
than a dozen major verion changes in the Window OS itself. Windows 95
to 96 to 97 to 98 (yes there was a Windows 96 as well). Then again,
Windows NT to to Window ME to Windows 2000 to Windows XP to 2003 and
soon to come up with Longhorn. Lets for a moment foget diff OSs and
just say that these are major version changes of the same software.
Has this chnage over really taken place in reality? No it has not.
Most of end users still use Windows 98 (despite MS's announcement to
end supporting the OS). Windows ME is used only by the people who were
unfortunate to have got the OS preinstalled on their laptop.

A server platform switchover is even more unlikely. I see a lot of
Windows NT 4.0 still running. Simply becasue, organizations cant
afford to make frequent changes to their "infrastructure*. Once
installed it will remain there. Add to that the know how. New
technology also means for them training the staff (and staff in all
parts of teh globe, doesn't welcome a change whole heatedly, you
leiterally have to push them). Investment in a new techgnology is one
of the most carefull made decisions. Larger the impact on the
organization, the more the debate.

Clearly, the life span of a software installation at the real site is
much more than it is at the software vendor.