Typically a week or two at most, but there were outliers. Everyone’s recruiting story is different. And this is a strange year, so who knows, but I wouldn’t assume an offer necessarily would come before September or even into October.
I probably don’t need to say it, but your son should be working now to email every school he has any interest in that will be at HF. In most cases, my son’s experience was that if he didn’t hear from schools before HF or SB, he didn’t hear from them after. He generally knew going into those events which schools were potentially interested and had some correspondence with their coaches beforehand.