Burned By Gmail Outage? Google Will (Almost) Buy You a Postage Stamp. Apparently the SLA for Google Apps will get you 3 days if uptime dips below 99.9% for the month. At $50 annually for the service, that's $0.41. (Google has decided to pay out 15 days credit, anyway.)
Service credits from the SLA:
Monthly Uptime Percentage Days of Service added to the end of the Service term, at no charge to Customer < 99.9% - ≥ 99.0% 3 < 99.0% - ≥ 95.0% 7 < 95.0% 15
That's $0.41, $0.96, and $1.91 credit for over 43.2 minutes, 7.2 hours, and 36 hours of downtime, respectively. I realize that across an entire business, that could potentially be a thousand credits or more, but what business would see a $1.91 per user credit as adequate for 36 hours of downtime in a month?
From these two register articles, Google's email service goes down and Google blames Gmail outage on data centre collapse, it looks like the downtime was about 2 hours and 15 minutes, but there are some reports of outages as long as 4 hours.
There is a nice article on High Availability on Wikipedia to compare uptimes on a weekly, monthly, and yearly basis.