This is a really relative problem.

In my experience it totally depends on the rack and the panniers you use. I was using Banjo Brothers Saddlebags for a while, and even with my short dogs (size 9) and 425mm chainstays I was getting heel strike. I upgraded to Seattle Sports Rain Rider pannier and I have like 4-6" behind my heels!!!! I can mount them further back because of the great system, and they're not real HUGE panniers to begin with. They can hold lots, but I typically cinch them down with my average 20 lbs. commuting loads.