I've also seen people make the other mistake, though. I've had people screaming about 2 hours of downtime violating our 99.999% SLA, apparently not realizing the five nines means the thing can be down eight hours a year (an entire business day) and still be in spec.
That's not true. 5 nines, ie 99.999% uptime means that the maximum downtime in a year is about 315 seconds:
60x60x24x365 x (1 - (99.999/100)) = 315.36
Wikipedia confirms: https://en.wikipedia.org/wiki/High_availability#Percentage_c...
So yes, 2hrs of downtime would most definitely violate the SLA. The 8 hours you refer to is the figure for 3 nines.
I often see it get written into contracts with very little thought as to what it means. You get some great looks from managment when you break the figure down into minutes though, especially when you don't have 24/7 staffing. A poorly timed cosmic ray could ruin your SLA.
* It would where I'm at now, because someone thought it would be a good idea to handle a critical business process by emails csv's.