There is no standard for determining a device's MTBF ... every manufacturer can calculate/extrapolate this value the way he wants and assume ideal conditions for the drive (humidity, dust, temperature etc.)
IIRC, Plextor stated that their Premium's MTBF is around 60000 (6.8 years) hours total - with a daily power-on cycle of continous 8 hours and 10% drive operation ... from my calculus, this leads to the following result:
Assuming 60000 hours total MTBF, you have to calculate the drive's total power-on time which is
60000/(24/8) = 20000 hours
The total operating time is 10% of the total power-on time
20000*0,1 = 2000 total operational hours (which should be a realistic number for drive life expectation) ... transposed to a DVD writer with the same MTBF this would mean that you should be able to burn 8000 4x DVD's before the drive kicks the bucket.
The problem is that no one can calculate the impact of different environmental conditions or a higher (or lower) daytime power-on cylcle so MTBF values are to be taken with some grain of salt ...
As being stated before, drive cooling may extend life expectation ... as well as a no-smoking area.
AFAIK, the read process will put less stress on the laser unit due to lower laser intensity ... but spinning and slowdown/focusing during error correction might have it's (unpredictable) negative impact on the drive's lifetime.
That's why I own several drives - each one with a more specific task. My 2500A is for burning only (since a DVD-ROM costs me some 30â‚¬, I'm willing to let it die before my 90â‚¬ DVD writer) ...
[Edit - Typo]