well now that negi's party is finally about to attack Fate's place and then go home (and save a world in the meantime) i estimate that there's only like one day left until they get back. It might become 2 days if they go get some decent sleep first and finish a random 'side quest' though. On the other hand, this upcoming day is going to take at least 40 or so chapters, with at least ten of them constituting some major battle. This last arc will then be as long as all of the other arcs combined.
however, we still don't really know what Fate's goal is. When negi and fate met just as they were going to the magic world, it's clear that fate didn't anticipate their meeting together. But if this is so, then they couldn't have gotten Asuna and proceeded to wipe out the world. It's only if fate knew that negi and asuna would eventually come or already had gone to the magic world that his plan (the world rewritten one) would work, and that he didn't anticipate meeting negi so early.