Indefinite. The old forks often had alloy crowns to and the carbon was wrapped up over them and quite thin. This construction method makes the transition from the crown to the thicker leg a stress point and I have seen several older carbon forks blades shear off at that junction. I wouldn't trust a used carbon fork with alloy steerer. I road a 350 g easton SC90SL carbon fork, and yeah it was more flexible and didn't feel as solid breaking, but I trusted it and it saved me about 500 g and that makes a difference going up hill.. it road really nice too when I wasn't out of the saddle on a steep grade. Descending was fine too.