∞
@Mtt Here's part of the definition from the RFC:
The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists.
I wouldn't say it's the equivalent of failure, a resource can have gone missing mistakenly or have been deliberately deleted. Or it might have never existed in the first place (a mistyped URL). Avoiding broken URLs is a noble cause and something I strive for myself. But when something is gone for real or never existed in the first place, 404 (or 410) should be returned by the server so that humans and robots, like search crawlers, can take necessary actions.