Phil, I've just swapped the orginal set out, after 8 years. I guess it depends on when your bike starts showing symptoms of them failing.
My bike started playing up two years but I could'nt figure out why - stalling when slowing down / chagning down at low revs.It also sounded slightly uneven on tick over
I had a dealer look for the problem (which they could'nt find) down load the latest program and balance the injectors, problem seemed to go away. When it started to play up again last spring, I had Bulldog's service the bike and the problem seemed to go away again. It was'nt until I read a thread on here that I put two & two together