Just an update for other new players who might be interested. This was just announced:

2.5 - 20th Jan
2.55 - late March

Give it another two months so the expansion should probably hit in May or June.

Unless you really play very casually (like two or three hours per week), you should be able to experience at least the full main story plus all the side boss fights and 4-player dungeons. Raids take more time investment and some people prefer only to raid with friends so that side of things really depends on the player.