At the start of MoP beta, the only things that were available were Wandering Isles and the Temple of the Jade Serpent instance. They then released a new zone every few weeks. There is actually a very good reason for gating out the beta content like that that may not even have anything to do whether it's finished. It's basically to force people to spend time actually testing each zone. If they opened up the full continent at once and let people level to 100, most people would just power level as fast as possible directly to the end game. With one zone open at a time, they force people to actually spend time in it and give proper feedback instead of rushing through it.
It's the same reason they did not provide premade level 90s at all for raid testing in the MoP beta. When they had pre-mades in previous expansions, most people would just make a pre-made and ignore testing the quests/leveling content. The flip side is that not providing pre-mades slows down the beta cycle and slows down raid testing, because to get a decent enough pool of people able to raid test, they need to give them time to get a toon to max level. It will be interesting to see if they do that again this time. If they stick with the no pre-mades strategy, you can almost guarantee that beta will be at least 5 months in duration. Another key test on predicting release date will be to see when they start raid testing. For the last 2 expansions as well as the last 2 major content patches, raid testing has started about 3 months prior to release. If they want to have a 4 month beta or something, they need to start raid testing in the first 4-6 weeks of beta. if not, it's a very clear sign that we are looking at 5+ months.