There was some discussion on this topic sometime back in this thread:
http://originaltrilogy.com/forum/topic.cfm/Implied-starting-date-of-the-Empire-from-OT-dialogue/topic/17700/
That thread focused on the OT, though, and didn't really delve into the EU. IMO, the early EU was somewhat inconsistent in regards to the timeframe of the Empire's formation. Some writers -- like Zahn -- went with a c. 50 BBY date, while others seemed to go with a shorter c. 18 BBY date.