回複：請教各位：關於父母B2簽證多次往返

I believe how long they can stay is granted by the immigration office at the port of entry. There is nothing you can do except that you file the request to extend the stay. Mostly they will give B2 parents 6 months and another 6 months extension. But they have to bear the risk of being rejected next time to get the B2 visa.

And, their re-entering US might be questioned by the officers, esp. at the edge when the B2 is gonna expire.