from these facts:
-> blizzard have officially stated they 'expected' further sub drops
-> wow census shows crash in activity levels over q2
-> going into exam and summer holiday season
-> age of wushu and other fp2mmo / league of legends growing very quickly in the east
and the anecdotal evidence of widespread dissatisfaction / inactivity, that people are pretty bored of mop now....
I would guess at least 500k drop, probably around 800k, but possibly much higher