Google pushes for 99.99% uptime for Google Apps, removes downtime from SLA
Google has also chosen to amend the SLA so that even downtime that's shorter than ten minutes is going to be counted. That was not previously the case.
In 2010, Gmail was up for 99.984% of the time, for both consumers and businesses. This translates into an average of seven non-consecutive minutes of downtime per month last year.
Google says that this downtime compares very favorably to on-premises email systems -- as you can see in the graph above. The numbers are from research done by the Radicati Group, which found that on-premise email averaged 3.8 hours of downtime per month. Of course, the uptime of on-premises email systems depends on many factors, and your particular system may work (a lot) better, but the point Google is obviously trying to make is that cloud solutions for businesses' email are at least good enough, if not better, than on-premises solutions, at least where downtime is concerned.
Along with sharing this information, Google has made its intentions clear: to make its email service "as reliable as the dial tone", which is usually four nines, or 99.99%.