You've got some good mileage on them already but it really depends on when you feel it's time. How risk averse are you. There are no "wear markers" like on Conti's and any tire will wear somewhat square, but I think given that Conti clinchers begin life with a little bit of an egg shape versus the Veloflex which are more round, that you might expect a bit more squareness on the Contis with wear, but that depends on pressure too. Or, just ride 'em till your next puncture and then bin 'em.
Here's a Veloflex Carbon tubular that I destroyed in an emergency stop when a pedestrian stepped out in front of our group and the guy in front of me slammed on his brakes and I had to do same. Didn't realize what had happened at the time until after lunch when I got back on and noticed a bit of a lumpy ride. Held till I got home and was promptly replaced. Almost a brand new tire as well. Oh well... I think this pic is good for showing the various layers in a Veloflex tubular, and it's probably similar in their clinchers I would guess. I'm thinking that last layer shown is the very last before the latex tube...
Also, here's what I found stuck in an almost new Arrenberg after a ride and was sure that upon removing a burst of air would follow. But nope, even though there was probably as much metal length inside the tire as you see outside, it must have slid in between the latex tube and the casing without puncturing. Amazing... wonder if a butyl tube would have been so lucky. No idea.
Anyway, I'm really impressed with not only the ride of Veloflex but with their durability as well, at least when compared to something like Vittoria which, for me at least, would cut up every time I looked at them sideways. In winter I've been using Conti Comps but take something else as a spare. Would hate to try and stretch a conti tubular (even a used one) on a rim when my hands are freezing cold and wet. I don't even like doing it when they're warm and dry, but I do like the Comps in the foul weather.