Most discs burned with the same drive

I am over 3000+ burns per drive, that is each of my 8 drives have burnt more than 3000 DVDs, (7xpio108 and 1 NEC 3500) and am wondering if they might start dying soon (they are still perfect in the mean time, althog the NEC has started showing some wierd signs, but its with regards to reading and not writing).

Does any one know what the life expectance for a basic DVD-RW is?
What is the most YOU have burnt with the same drive?
As there are some many mechanical parts there is bound to be a limit: 5000?, 10000? 20000? more?

I dont want to be supprised when I suddnly have to replace all my 8 drives at the same time, so if I anticipate before hand, I might budget the cash.

Most drive manufacturers list an estimate of the hours that the drive will work, ie: 60,000 hrs, 80,000 hrs. etc… So figure how many hours of burn time. Who knows, some drives may be like the energizer bunny, and go forever, and some may just get somewhere close to the estimated hours.