I had a .308 barrel (Bartlein 5R) that was shooting cleans with excellent X-counts at 600 yd with ~3000 rounds through it. In the last 4 matches I used that barrel, it gave scores of 200-12X, 200-11X, 200-12X, and 198-8X, so it was shooting very well. I could feel it start to get a little rough in the throat with a cleaning patch and you could plainly see firecracking with the borescope at about 2800 rounds. I finally took it off at just over 3100 rounds because I had another one ready to go.
The type of powder you're using and how hot your loads are can make a big difference, but I would generally expect about 3000 rounds out of a quality .308 Win barrel. Of course, if you're running 200 gr bullets at over 2700 fps, or 185s at well over 2800 fps (as I know some people like to do), then noticeably shorter barrel life is to be expected. However, if you're not pushing your loads that hard, you may well have another 1000+ rounds left in your barrel. If you can feel the barrel starting to get rough in the throat with a patch, it's not uncommon for it also to start coppering up faster, especially near the muzzle. You can smooth out the throat by giving it 60-80 strokes in that region with some KG-2 and get perhaps another few hundred rounds out of it if you don't have a new one in hand.
Realistically, having a new barrel blank in hand isn't going to hurt you and is probably a good idea. I generally have two cut at a time, simply because you never know for sure exactly when one will crap out. If you can find the exact blank you're looking for in stock somewhere, the lead time is not an issue. However, if you want a special contour, twist rate, etc., and have to order direct from the manufacturer, it's probably safer to count on about 6-8 months lead time.