I'd say the absolute worst-case for data loss would be 15 minutes, but that's assuming that the transaction log is totally destroyed and unable to be recovered, and your log backups are on a disk or tape that is not affected by the disaster.
The far more likely scenario is that you'll be able to take a tail-log backup and restore up until the point of failure, making your probable data loss considerably smaller - much closer to zero.
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Bob Pusateri
Microsoft Certified Master: SQL Server 2008
Blog: The Outer Join[/url]
Twitter: @SQLBob