I haven't seen an official date, but if memory serves it takes place during the old republic, approximately beginning around Episode I. The Outbound Flight Project was originally referenced in the Thrawn trilogy in 1991, and it was said that a group of Jedi went on an exploratory mission into the Unknown Regions. I think the Darth Maul novel (Shadow Hunter?), which takes place very shortly before Phantom Menace, also starred a Twi-Lek Jedi and his Padawan. In one of their Jedi Temple scenes, they briefly encounter some of the Jedi preparing for the outbound flight project.
At least that's how I remember it. At any rate it shoudl take place smetime between 20 and 35 years before A New Hope.