The short answer is: usually a very long time, provided of course that you buy quality poles and that you don’t abuse them.
My personal pair of Swix CT2 poles, with the Twist&Go tips have been in nearly constant use for over a year now. They are very light and very rugged.
I have used them for more than classic Nordic walking – have taken them for a number of hikes in the mountains, where regular hiking poles would normally be used.
The poles have a few dings and chips in the finish and the grips and straps do show a bit of wear, but after over 1,300 miles (as of November 17, 2006) are still in fine shape. Just take a look at the photo, which was taken today.
The Swix CT2′s are made from 100% carbon fiber, a very strong, light and resilient material. Although I have heard some reports of carbon fiber equipment being sometimes a bit brittle, that hasn’t been my experience at all. My poles have taken some inevitable pounding on different occasions and after long, hard use appear to be as sound as ever.
Take a look at an earlier article on the subject.