By that metric WoD was in development for ~26 months and Legion and MoP for ~22. In other words at a loss of four months.
Not really sure you can make any sort of actually meaningful statement on expansion size or quality based on the time period between one expansion release and the next. It's about how the time is utilized and divided.
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I really do not see them announcing 11.0 before or right on top of 10.3 announcement. I guess the other-other option, as unlikely as it would be, would be them announcing 10.2 in the next couple weeks and announcing 10.3 in like September, before 10.2 even comes out. Basically move the problem forward so that instead of 10.3's existence being conceptually cannibalized by 11.0, 10.2 is cannibalized by 10.3, to a lesser degree, since they are both just patches.