My guess: We will never find out.

The way things have been going it's very hard to predict. ZOS have set themselves a schedule of 4 releases per year - 1 dungeon DLC, 1 PvE DLC, 1 free update and one 'chapter'/expansion. But this must be at least the 3rd time they've changed their release schedule/business plan so it's difficult to know how long that's going to last.

At the very least the chapters and PvE DLC should include new maps, but I don't remember them explicitly saying that, they could easily release new quests (maybe with a related skill line and/or faction) that use the existing maps and call it DLC. They could also have a full DLC in a relatively small space like the Imperial City, or one that's not on Tamriel at all like the upcoming Clockwork City and they could even have a full chapter that's set in Oblivion.

They may also be wary of showing us some/all previously unseen areas of Tamriel because Bethesda is probably working on a 6th single-player TES game and although that will probably be set 1000+ years in the future Tamriel shows remarkably little change over that time so if the same area appears in ESO they'll have to coordinate between the two studios to make sure it's reasonably similar, which adds a lot of work (or worse, meetings) and limits creativity for both studios.

On the other hand I'm sure they know that new maps would be a major selling point, especially (if they're set in places we've never been...or not since Arena anyway) with those of us who were a bit turned off by their "Look! You can re-visit all the places you've already seen! OMG nostalgia!" approach to Morrowind.

So, like I said, it's hard to predict. Too many factors and too many unknown decision making processes.

But I'm hopeful that we will eventually get to see all of Tamriel, and Akavir and beyond.

My guess is 9 years, 7 days, 15 hours, 17 minutes and 37 seconds + x from now.