I only got maybe a year out of the old street (yellow) Konis I ran, was underwhelmed, but then I think I did unintentionally abuse them more than even normal for racing (rookie, what can I say!)
Now I'm on Bilsteins, and I can't believe the durability... now on their 6th year, and I finally opened the rears up to tweak them a little. On discussion with a fast local Volvo driver, seems that at very least to replace the oil and recharge the N2 charge every year is not a bad thing. I checked the charge on mine before disassembly, they were both down to anywhere from 70-100psi; they're supposed to be more like 200psi, apparently.
Gonna have to send my fronts in to be serviced this winter, I guess. Seems like a general rule of every other year, if you're using them hard (I usually have 6 race weekends per year), might not be bad timing for a rebuild... if you're only managing 2 weekends a year, stretching them out to 5 years might not be the end of the world!