I asked this same question a while back. I have a bird that had 230,000 miles on it before swapping the heads from another bird. I'm going to get the springs tested in a few weeks to see the difference between the stock rating, 230,000 miles, and 140,000 miles. Hopefully that will shed some light on the life-span of these springs.
btw, I ordered a set of replacement springs from SBI through an engine builder for 34 bucks. not a bad deal. but, we'll see how long they hold up.
also, i've heard on here that springs will loose thier "strength" over time and not necessarily break. But I can only speak for 230,000 miles. after that much use, I dont' know.