So I'm approaching this question the wrong way. Instead of asking how long a road would last (which is apparently too broad for stackex), let me tell YOU how long I want it to last and you can tell ME under what conditions that would be reasonable.
I'm thinking I want a road that if looked upon would only reveal patches. Let's call it somewhere between 10-30% of the road remains. I would prefer if the road were previously cut through a heavily wooded, back-country area and over time the grass, leaves and trees have encroached (actually, the trees probably approach but don't encroach). Yearly temps range from low in the 30's in winter to highs in the 90's in summer. Average rainfall somewhere between 20 - 40 inches per year. The only wear comes from sun, wind, rain and infrequent snow.
Based on the above criteria would it be reasonable for such remains to exist after 100 years? 200? 250-300?
What parameters above would you change to MAKE such a road last 100 years? 200? 250+? (If this question is too broad for stackex, I'll withdraw it)