How Long Does The Average Disc Burner Last?

How long does the average disc burner last? 100 discs, 200, 300, or maybe more? Is it a matter of laser degradation? Can it last years? How do you know when it's time to toss it out for a new one?

 

I've burned a lot of disks, way over 300, after 4 years and also being a heavy smoker which obviously ain't good for either of us. How to tell if it's bad? When it continuously burns coasters or no disk present, repeat disk errors at same point when loading OS or programs from known good disks. Others will probably have better tips.