99.99%. It’s a compelling figure. The uptime guarantee trumpeted in SLAs the world over.
Of course, 100% is simply unattainable. Those SLAs must provide wiggle room for the .01% of unplanned, unavoidable, ‘well I didn’t expect THAT to happen, ‘what do you mean there’s been a power spike to the UPS’, ‘so THAT’S what that switch does’ events.