How long does it take to clear border controls when arriving by ferry at Rotterdam from Hull?
If you are one of the first of the ferry, a few minutes, if you have to line up at the end of a long queue, half an hour to one hour. Mostly there are not that many foot passengers so that is the shorter time.
In rare circumstances longer is possible.
If you travel by car and need a speedy departure, talk with the boarding staff so you can get a place on the deck that unloads first.