How long can a DC be offline? https://social.technet.microsoft.com/Forums/windowsserver/en-US/7ffe5c2d-3a20-4ede-8499-2688f4dc3719/how-long-can-a-dc-be-offline?forum=winserverDS
60 to 80 days depending on OS and SP level. It is ultimately determined by the 'tombstone lifetime' in the forest. That being said, from personal experience, the longest we've ever had one offline is a couple days.
http://technet.microsoft.com/en-us/library/cc779177%28WS.10%29.aspx