Google has set a pretty high bar for itself, guaranteeing 99.9% availability of Gmail, Google Calendar, Google Talk, Google Docs, Google Groups and Google Sites to its Google Apps customers. If it cannot meet that level of availability, all of its customers are entitled to a certain amount of free days worth of service in each billing cycle.
In a somewhat self-aggrandizing blog about Google's cloud services today, Google Apps product manager John Collins reiterated that Gmail was up for 99.984% of 2010, and revealed that it's currently at over 99.99% uptime for the first half of 2011.
Looking through the Google Apps Status list reveals how comparatively isolated Gmail outages have been. On August 29th, for example, there was an outage for 3 hours that affected less than .05% of the Gmail userbase, and the last outage was a little over a month prior, affecting less than .01% of the Gmail userbase for about 8 hours, by the unofficial number of registered Gmail users, that amounts to less than 20,000 users.