I would wipe it down quickly after each use, then it will never have the chance to get real dirty. It will also be easier to clean before the food has a chance to dry!
I usually clean the highchair when I’m doing my weekly cleaning. The highchair tray is cleaned every night when I clean the kitchen, but I do a massive cleaning of the entire house once a week, and the highchair is involved in that.
I usually clean Emma’s after every meal with a soapy wash cloth and then on the days I vaccum I use the hose to sweep out the seat and behind it.
Do a quick wipe down after each time after he eats. Then a sanitize ever ounce a week.