Most webpages die after a couple of months. The average lifespan is something like 100 days. That's longer than it used to be. In the late 1990s, the typical webpage lasted for around 44 days.
But quantifying pieces and pages of the web gets murky pretty quickly. For one thing, finding a reliably representative sample of links for this kind of analysis is tricky if not impossible. There's also a sort of existential question, as Nicholas Taylor wrote for the Library of Congress in 2011: “That is to say, we take for granted that we know what it means that a webpage has ‘died.’”*
In other words, what lives and dies on the web depends in large part on how you define a webpage or a website in the first place. “Is a ‘webpage’ defined by its URL or by its contents?” Taylor wrote. “A non-resolving link doesn’t necessarily imply that the content once hosted there no longer exists; it may have been archived or simply exist at a new location (albeit, one mediated by a paywall) to which the web server was not configured to redirect page requests. Conversely, a resolving link doesn’t necessarily imply that the same content is still hosted there as it once was.”
But if you just want to get a sense of how big the web is, how much it has grown in 20 years, there are blunter measures. In 1994, for example, there were fewer than 3,000 websites online. By 2014, there were more than 1 billion. That represents a 33 million percent increase in 20 years. That’s nuts!