Well depends, if is like last year they will know on January, Sean said it that last year they knew back then.
TCAs is usually were most networks announce what is renew, ABC with older president was the only one that announced in May.
Another way of them knowing, is if the cast wants out to do other things, so in that case they know already that this is it, JMo even said last year that in her opinion s6 was last and they only have contracts until this year, all depends if they want renew or not.
Also , so many cons and Robert doing a con when he hates them, is suspicious.
If they will get a s7 I see a season with like 10 or 11 epis only.