I’d allow a 60 min queue for JNB International immigration. You only need a couple of 380s and the odd 744 or 777 to fill the hall (this is based on mid arvo arrival). And the staff seem unable or unwilling to stop people from cutting the queue - and then there are those who walk to the far end side and are curteously directed into the front main line because they made an “error” (sigh).