.22 Long Rifle: The lowest estimate I've ever seen is 10,000 rounds. I've seen some estimates go as high as 500,000. I think the question here is academic; you're not going to wear one out, period.
.224 centerfire: Probably a maximum of 4,000 in small rounds like the .222, to a minimum of 2,500 in big cartridges like the .22/250.
.270 Winchester: 3,000 or so.
.30/06: I recall seeing 5,000 somewhere respectable, maybe in Hatcher's Notebook. I'll go with that.
7mm and .300 magnums: 1,500 to 2,500.
Great Big Magnums (.30/378, etc.): 1,500.