[MENTION=129665]epnnf[/MENTION], time for you to get a copy of "The Bicycle Wheel" by Jobst Brandt. Read it a time or two, pick up a truing stand and a spoke wrench, and fix the wheels the "expert" screwed up before they break. You'll save money within a few years.

To answer the original question, on average every 6-7 years. (I'm still learning!) I've replaced one rim whose brake track wore out, another lightweight rim after I found some amazingly bad expansion joints that were closer to curbs, and one wheel with I failed to keep the bearings clean and greased.