Hey Brian,
Barrel life here is really up to you. Not trying to dodge here, but there's so many variables involved here that you're the one who ultimately has to make the call as to when a barrel is shot out. Certainly, going with a badly overbore (extremely low expansion ratio) cartridge will tend to aggrivate the problem, but that's not the only consideration involved. Accuracy has always been the deciding factor for me, and it's important to understand how a barrel "goes bad." They don't just quit shooting, they die slowly. Most will continue to shoot outstanding groups . . . with the occasional flyer. Those flyers will become more frequent, and stray further from the group over time. It's up to you to say when it just isn't worth the hassle for you. For a benchrester, that won't be long. For a Highpower shooter (shooting unsupported and usually with iron sights), it may be a bit longer. For a Silhouette shooter (all shots fired offhand) it may be a while longer before he even notices the slight degradation in accuracy that marks the end of a barrel. In short, they'll all have different answers, and that's fine. For a varmint shooter (which is what I'm assuming you're doing?) it'll depend on how many times you can stand calling a shot a good hit, only to see a clean miss and a dog scampering back down his hole.
What you're shooting, even within the same cartridge, will also have a profound impact on this. Heavy bullets give better long range performance, but at a price; they're harder on barrels than lighter bullets are, all else being equal. Still, if it's the long range perfortmance you're after, that's the price you'll pay to get it. Said it before, but we're right back to the no free lunch deal. Best advice I can give you here, is to talk to guys who're doing the same sort of things you want to do, and with similar cartridges. I'm sure they'll be able to tell you how things have gone in their rifles, good, bad and indifferent.
Hope that helps.
Kevin Thomas
Lapua USA