Im guessing if they keep their word, 5.3 wil last about 2 months, so end July will be 5.4 that goes around 4 months. Around the end wil be just when blizzcon is going live, opening blizzcon will show new x-pack 5.5 will be around choosing new warchief and setting up 6.0 .
The last time they didnt even need a blizzcon to announce the new x-pack and shorty after they setted up the beta, so yeah its possible to start beta in the beginning of december.