I have spent my life in pursuit of soft surfaces and wouldn't be running without them. Deciding to live somewhere suitable is one of the best decisions a runner can make.
20 years ago I moved to London and vowed to stay within half a mile of Wimbledon Common; I've managed it so far. We have two huge parks and the Thames towpath all connected, another one (Bushy Park/Teddington) nearby. Anyone who can run in the daylight can get 60-80% of their miles off road. This corner of London is therefore home to numerous running clubs, many elite athletes, and the Kenyans and Australians whenever they are over here.
As a younger runner the best I could hit was about 30% soft surfaces; where I grew up was countryside, but most of it fenced off, and where I went to college it was typically 2 miles out of town to the open country. I hit my limit around 50-60mpw above which I'd get injured pretty fast. Moved to the soft stuff and 70 felt easy. (Of course now I have Letsrun and know I should have shot for 100...)
I think a more important factor than the bare percentage is where you do your fast reps and tempos. The pounding from banging our hard reps on the road is way greater than from doing an easy 2 mile warmup on the way to a park. A lot of my aches and pains come from hard road runs or long reps on road. Many top athletes have made a point of doing their reps on grass in winter, rather than track or road. Mick Woods' squad at Aldershot apparently have floodlit playing fields for intervals on winter evenings.
Then again, I know a few people - usually very skinny marathon types - who can knock out 100mpw on roads without problems. But only a few.