The last major update, when the 4 shards Serendipity, Sacrifice, Patience, and Void were added, along with some minor changes to old shards and a few fire cards, took a couple months from announcement to implementation. The major update before that was the arena, and that one was under discussion for awhile as well. The other updates I've seen since starting (last March) were things like bug fixes or minor cosmetic/convenience improvements, and those generally appeared right away or within days after announcement.