Microsoft offers a financially backed 99.9% service uptime guarantee for Office 365. "Uptime" is the amount of time a cloud-based service is available to the user and this translates to about 8 hours of downtime per year.

A full copy of their Service Level Agreement (SLA) can be found here.

While Microsoft offers an industry-leading guarantee for their service, they have actually exceeded their promise in the past fiscal year.  In Fiscal Year 2013 (June 2012-July 2013), Microsoft's actual worldwide uptime was an astounding 99.98%, 99.98%, 99.97%, 99.94% and 99.97% per quarter for the service.

Furthermore, Microsoft is committed to maintaining transparency and discloses quarterly uptime numbers (as well as a variety privacy and security information) on their Office 365 Trust Center website.

To Download Microsoft's SLA, please see: Microsoft Service Level Agreement (SLA)

To see the worldwide uptime visit the Office 365 Trust Center