We had the cap shingles on our gable roof redone this summer. We installed a 3-1/2" zinc strip (with 3/4" covered) sticking out from the cap shingles (i.e. 2-3/4" of exposed zinc). Too early to tell how well it works *but* I must say that the roof has always been cleaner "downstream" of the other metal flashings on our roof (skylights, chimney, stink stack).
We're in the Pacific Northwest so zinc strips are pretty common here--seems like they provide about 8 feet of protection (downslope).