Just googling, here is a couple stats.
HP dvd840 LightScribe 16X DVD:
Mean time between failures (MTBF): 100,000 POH at 20% duty cycle in room temp
MTBF: 60,000 POH, Operating Duty Cycle (Read) 20% POH, Operating Duty Cycle (Write) 2% POH.
POH = Power On Hours
I gues it really doesn't matter as they're $30, but I'm pretty sure I've already burned thousands of DVD's at a slow 4x (about 15 minutes to completion).
Let me estimate 1500 burned DVDs at 15 minutes each = 375 hours.
2% of 60,000 = 1200 hours
But leaving the PC on 24/7/365 is only 8760 hours a year, meaning it would take 6.85 years to reach MTBF of 60,000 POH.
As I've only had this PC for 2 years x 8760 = 17,520 hrs.
17,520/60,000 = 29%
375/1200 = 31.2%
So I've used my write function more than I should have, going over the write duty cycle by 2.2%.