The short and rude response is: 10% services has been radically redesigned or dissapeared within two years since launch. 20% within the next two.

That is, from own research. But after filtering out Marketish exclamations and opinions wandering in void, there are still interesting information on the issue to be found in the net.

First of all - the methodology applied. Some researches consider the page dead once it disappeared from its address. Average lifespan measured according to this criteria is below 100 days! Another approach is a business perspective, considering www service alive as long as it maintains the characteristics of its target, services and information it provides. In the later case lifespan can be stretched over a decade.

These are certainly extremes. Here you are the practical definition of the end of the webpage life (or the webpage life cycle): it is no longer visited, or the issue of its revitalization appears on the management agenda - and in the budget. In this sense the average lifespan can be estimated between three and five years, inclusive.

It is worth to be aware, that there are two different areas influencing webservice life time. Most important is the stability of the organizational entity the webservice belongs to, and circumstances in its environment (i.e. legal, or business). Only if, and once these conditions sine qua non are favourable, the technical and aesthetical issues step up to the front.

Daily maintenance of well designed service need not to be time consuming nor expensive. Regular content updates and relatively small changes of the presentation layer can ensure feeling of freshness and stimulate interest of visitors. „Three to five years” should be perceived as a borderline, after which the web service needs not necessarily undergo a groundbreaking overhaul, but at least a serious evaluation.

For those interested (I hope these pages have not ended their life cycle meanwhile):
Mike Ashenfelder: The Average Lifespan of a Webpage
Keith Darnay: A look at website lifespans
HirMon: Website Life Expectancy
Illustration: Hans Baldung „The Seven Ages of Woman” (1544)