Our iMac history:
17" G5 iMac, bought 12/04. Ran 6 years then power supply failed (the defective capacitor problem). I gave it to a friend who replaced all the capacitors to get it running and then gave it to his lady friend.
20" C2D iMac, bought 11/06. Display failed under Applecare. Hard drive recently failed (I had given the system away).
24" C2D iMac, bought 7/08. Used by my (late) father, now my daughters computer. No repairs.
24" C2D iMac, bought 6/09. My wife's. No repairs.
27" i7 iMac, bought 11/09. No repairs.
I'd guess a 5 year life based on the older 3 systems. I don't consider that particularly good. I'm still using an 8 year old Dell (no repairs) and had two others that I kept using for 7 and 8 years with no repairs. But they run much cooler. The heat kills.
17" G5 iMac, bought 12/04. Ran 6 years then power supply failed (the defective capacitor problem). I gave it to a friend who replaced all the capacitors to get it running and then gave it to his lady friend.
20" C2D iMac, bought 11/06. Display failed under Applecare. Hard drive recently failed (I had given the system away).
24" C2D iMac, bought 7/08. Used by my (late) father, now my daughters computer. No repairs.
24" C2D iMac, bought 6/09. My wife's. No repairs.
27" i7 iMac, bought 11/09. No repairs.
I'd guess a 5 year life based on the older 3 systems. I don't consider that particularly good. I'm still using an 8 year old Dell (no repairs) and had two others that I kept using for 7 and 8 years with no repairs. But they run much cooler. The heat kills.