I think the time between 5.4 and the next expansion will be alot shorter than before because:
Example 1: They have larger team working on WoW than ever before
Example 2: The last tier patch released 2 months before the expansion announcement. Dragon soul didnt release until around 1,5-2 months after blizzcon (2011)
Example 3: as example 1, cuz of the larger team they can do fix more of the features at the same time than before which can speed up the launch of beta. I do think that the beta will take some time to do but till come alot earlier than the MoP beta did.