"as tall as the chips go" on your car and based on how well your tires can throw rocks - that's my answer to your question.
Experimenting with spats, my 1st set was too short. My 2nd set was still too short. The 3rd set was jussssssst righhhhht. Part of it depends on how much flare you have covering the front tire (how much the tire sticks out) too and if you run hard or stickier tires on the street. The stickier tires definitely pick up and throw more pebbles back and up the side of the rear flare. You'll notice the flare on an SPF covers more of the front tire, where as other cars have a different cut-in shape and more front tire is exposed. This is a factor, or at least on the cars I've earned chips on.
When I ran my Goodyear Billboard tires, chips were getting all the way up the side of the flare just parallel to the top of the door.