Speaking as an IT
wonk, I can tell you that a number of companies (like airlines) strive for what's called five-nines for their mission-critical technologies. That means that these systems should be 99.999% available or reliable, depending on the measure that the company uses.
On average, this standard means that systems should not have unplanned downtime of more than 5 minutes, 15 seconds per year.
Obviously, maintenance, upgrades, and other work can bring a system down while the work is being performed. That's why the standard almost always mentions "unplanned downtime". The half hour that the entire thing is down to swap out a network switch doesn't count.
Companies spend millions upon millions to achieve five-nines, especially in industries like aviation, banking, and telecommunications. Airlines spend a virtual fortune on redundancy and resiliency, for good reason - no IP, no boarding passes, no departure, no revenue, and angry passengers. Even if the system may be slow, the system is at least up -- the standard generally doesn't say five-nines of great performance
Some achieve this standard, while others continue to throw money around. AT
&T, for instance, had a standard of six-nines of availability when it built its long distance network. That's a failure rate of one in one million attempts. Their internet service, though, is lucky to see one-nine of reliability.