LWN has run a few articles regarding "Enterprise" kernels that address your questions. You seem to be overlooking the fact that Red Hat releases an updated RHEL-5 version about ever 6 months with a new 2.6.18 branch... and in doing so they always back-port drivers, some features, and security fixes. In the last few updates they have also seen fit to re-base some desktop apps on newer versions... or upgrade an app if the upstream has discontinued the base they were using (take Firefox for example in RHEL4).
RHEL's 2.6.18 is really 2.6.18 but with a significant amount of code from newer releases mixed in.
So to answer your question... how long can they continue to use it? That is like asking how long people can live in a house? As long as that house is continuously remodelled to meet the needs of the residents, it can go on until the planet gets destroyed.