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most springs are just steel. Depending on the strength of the steel and the amount of flex they're under will determine the life before unacceptable margin of stress makes them un-useful. That is a very spring specific thing. BUT! The springs will most likely die long after piston rings or bearings fail. Diesels last what, 1-2-4 million miles? I don't work on industrial diesels, but I'm betting the springs in OTR trucks aren't ever replaced unless they are stretched or compressed out of spec.
A friend of mine does local deliveries, home every night; anyways, his truck just had its first serious engine service at 1 million miles. Sure it's had the weekly and monthly oil, fluids, leakdown etc tests, but never the engine cracked open. At the 1 mil mark they put new crank bearings in, new piston rings, new turbo seals, and put it back together (he's a car nut, so he asked the mech what was done). He says it runs a little stronger now, probably has slightly less friction and higher compression. Netted him about .3mpg more too.
Placibo? maybe, but the .3mpg is proof its running better. When you average 6.85mpg over 800 miles in a day, .3 is a huge savings.
Bottom line I'm saying: Steel springs last forever. Titanium lasts even longer (less pron to fatigue). Go with lighter springs if you want slightly better FE, but keep the rpms down to prevent valve float. Don't want any head crashes...
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