I'd give him as long as I said, all of next season and then judge from the season after. When you have a manager merrygoround at a top 6 club you end up having to try and shift on players that nobody else can afford and replace them with players that you're being overcharged for, so a rebuild takes a lot longer. Right now he's stuck with players he doesn't want from previous managers - sack him and the next manager will be stuck with players Ten Haag signed that he doesn't want, and so on, and so on.
At bare minimum you give him next season.