Technology Futurist:

"Since I saw Larry Borsato mention some figures in the 99.999% availability requirement-for-VoIP-networks debate, I decided it was time for me to dust off my networking theory book and implement Erlang's formula on an Excel spreadsheet using VBA, to get a better feel for how much each "9" matters.

Here are the results that I got:"

Availability Associated Downtime per Year
99% 87 hours and 36 minutes
99.5% 43 hours and 48 minutes
99.95% 4 hours and 23 minutes
99.99% 53 minutes
99.999% 5 minutes