Traditionally there should be no less than 1 cm. of clearance between tire and fender. I have used less but have run into frame/fork flex causing the tire to touch the fender. I've also used more but dislike the unsightly gaps.
I prefer to mount the front fender with a large rubber or leather washer and steel fender washer between fork and fender. I usually allow 4 mm for this. So figure total gap from bottom of fork crown to top of tire is about 1.5 cm.