I'd say 4-5 years with minor upgrades (e.g., more ram, new cpu, etc.).
I used a Pentium 1.4 Ghz for 4 or 5 years. During that time, I only added some RAM. A couple years ago I upgraded to an Athlon 64 3200+ on Socket 939, with the idea of upgrading to a dual core CPU later (after prices came down), which I did just two days ago - I changed the CPU to an X2 4400+ (cost me much less than the 3200+). I could have waited a bit more but Socket 939 CPU's were becoming very hard to find, especially dual cores. This setup will work for me for another 2-3 years. I only have to buy a new harddisk, and maybe some more RAM. The old Pentium 4 system is still being used by my wife without problems. It's about 6 years old now.
I also have a 4-year old Athlon XP-M 2400+ laptop that I use for my work, and I don't plan to get rid of it soon. I upgraded its HD and RAM a couple years ago so it's still going strong. And I do some pretty heavy math processing with it.
Of course, I'm amazed at what those new quad-core systems can do, but then again, I'm also amazed by my new dual core. It improves Sonar's performance by about 70%, which is basically all I care about. And for what I do, my current setup is more than enough. My most complex projects are handled well by this CPU. I don't see myself requiring 50 softsynth tracks and 100's of plugins. I don't mind freezing tracks occasionally, or upping my latency when mixing/mastering. In any case, it's good to know those new systems are there for people who needs/afford them.
As a sidenote, I don't upgrade Sonar each year, either. I'm still on S5SE and probably will wait until v8.