I always say allow at least 1/4". Sometimes that's not enough.

I rode over a section of road earlier this summer between Kamas and Park City that had been freshly paved and tar coated pebbles kept sticking to my tires and jamming in the brake bridge and fork crown. It was really annoying as it was lot of climbing and it would bring me almost to a stop, then I'd have to hop off, clear the tires and go again.