B-2 stay 6 month, too long?

my in laws just got denied in guangzhou for b-2 visa. when he asked: how long do you want to stay in usa? answered: 6 month, cause it's been long time haven't seen my son. then, he said: sorry, we cannot give you visa this time.

is 6-month too long? they say 1 month will be better? how soon they can go and get another interview?