BC Beta: 4 Months (Outlands, 2 races, 1 class)
WotLK Beta: around 4-5 months (Northrend, 1 new class)
Cata Beta: 6 months(Old world remade, 2 new races, LFR, Ext)
MoP Beta: 7 Months ( 1 new race, 1 new starting area, Pandaria, new class)
WoD beta:? (Draenor, Item Squish, Garrisons, 90-100 questing, New models ext)
Blizzard has gone on to say they do not want MoP to last the same or more then what Cata lasted to MoP.
Now Cata released Nov 7th 2010, Mop released 22 months later on Sept 25th 2013.
If WoD releases after Feb-Mar 2014 like say Sept 2014 it will make WoD release 25 months after MoP, but if they do release Feb or march that would put us at 18-19 months of MoP.
So if beta starts this month which i think it will. we can get around 3-4 months of beta in before the release, with there not being a lot of content like the past Xpac's this could be possible.