Hmm, good question. From previous posts we know that many on the C&V regularly ride 30+ frames and put some real miles on them.
My sense is that the overall condition of the frame probably matters the most. Does age in and of itself cause the frame to deteriorate (as opposed to the fatigue and stress of regular rides, which would subject the frame to weight, flexing as you make turns, road bumps, etc.)? If not, then a frame that is old should and still in good shape should hold up for a lot of miles. My two oldest bikes, 1975 Schwinn Traveler and 1979 PKN both feel as solid as my new Reynolds 520 steel Bikesdirect bike. The PKN has the best overall feel of the three.
I have ridden my PKN as much 150 miles in a week w/o any difficulty. The frame had some minor surface rust, which I cleaned up and repainted. My only limit on distance with the PKN is the tubular tires. I am afraid to get 50 miles out in the middle of somewhere and get a flat. I carry a can of Vittoria tubular inflating stuff, but I still keep closer to home.
I am not sure if I have really provided an answer, but I agree with previous posts that I own the bikes for the purpose of riding them and ride them I do
