This thread has inspired me to do the math because I've been curious, so here are two maps of the trip I did last summer:
The plan was to ride as much dirt (counting dirt roads as dirt)as I could, and by the end it felt like I had ridden a lot of freakin dirt.
The first map is of the plan, and it came out pretty close to what happened. The green line which I rode is mostly dirt, the red and black lines are truck by highway.
The second map I just made, represents the paved roads that I rode on. It really doesn't look like much. Visually, maybe 10% of the trip was pavement, and that's what I felt like by the end. It was like a 99% dirt ride!
The second map adds up to 600 miles of pavement.
The total ride was 2350 miles.
25% on paved roads
I conclude that to get 90% off road, a few things need to happen:
1: Have an awesome plan, and not much can go wrong. Four of those long sections above could have been planned away better, the rest are from things going wrong (forest fires) or not following the original plan (got tired and hungry)
2: Live or ride somewhere that has a lot of dirt roads that go somewhere
3: Count it by time rather than distance. I can go 100 miles on road without a pee break. 100 miles off road will probably include a lunch stop.
4: Your bike is going to spend some miles in a truck. The good news is that those miles don't count toward tires or oil changes.
5: Don't ever tally up your mileage breakdown unless you're ready for it