According to Leland Chee (Lucasfilm's in-house chronology expert), the Droids TV series exists around 15BBY, 15 years prior to the events of A New Hope.
15 BBY for Droids15 BBY for Droids has been in use since Insider #73 (2003) [See below], if not earlier.
3 ABY for the Ewoks movies have been in use since Behind the Magic (1998), if not earlier.
These two timeline positions supercede any timeline positions released before those sources. If you have an official source released after the publication of the above sources that lists a differerent timeline position, let me know.
Star Wars Forum - Tasty Taste (AKA Leland Chee)
The Droids cartoons take place 15 years before the events of Star Wars: A New Hope and center around the galactic travels of C-3PO and R2-D2 before they meet Luke Skywalker."