If you have had to deal with SLA (service level agreements) from your internal IT or an external vendor, the availability (uptime) is one of the first questions. This is a number usually presented as a percentage (usually 99.9%. Adding or removing a few 9 after decimal might not seem to matter much, but in reality it could. The table depicts the points: Total downtime (HH:MM:SS)

While a 99% uptime SLA might not be acceptable because it is still high (14 minutes downtime per day), paying for 99.999% uptime might be an overkill as well. The most common SLA numbers I have seen are 99.9% or 99.99% . Hysterically, though I have also seen 100% uptime SLA, but these companies are only asking for trouble for themselves, because I do not believe 100% uptime is practical even with every high availability algorithm and network architecture in place. Hopefully these numbers are useful when you have to decide next time.