I don't think there is an exact date, but yeah, its not something that opens immediately upon an expansion release.
The log in scroll says 24 of 27 and the last expansion I purchased was VoA way back when. Selo is in EoK now, expansion 24, and can zone in to it just fine. On January 6th Selo will hit TBL. Will be interesting to see if I'll be able to go there without buying it (well, buying CoV to get them all) or not.
Due to the excessive server issues on today's new expansion launch, it is confirmed they will be adding TBL to the free-to-play lineup.