What kind of shoes are they? I guess I'd be surprised if they're not minimalist shoes, if the soles don't show at least some compression after two years. My shoes usually have visible compression in the impact area (midfoot) after 150 miles or so. But if they're not even visibly compressed, you should have a long way to go with them. I replace mine when they start to feel sloppy. Usually between 400-600 miles. Although you shouldn't be relying on your shoes to support your feet, you'll know when they're not cushioning the way they used to. Depending on how heavily you run and how evenly the soles are compressed (I would think walking would mash them a bit more evenly than running), some people wear insoles out quicker than outsoles, so you might be able to extend the life of your shoes by replacing the insoles only.
I do track mileage but I just go by feel when to replace a shoe, but I guess when I replace my shoes, the outsoles are compressed by probably about a third or more from new. Maybe even up to 40% or so in impact areas, but definitely I wouldn't let them go past half.
Does that help?