I don't think there is any real limit other than when hardware malfunction happens i have an 5yr old MSI 8x dvdrw drive thats still kickin it and burning as good as the day it was bought and probably done 7K odd burns and i have had and Asus dvdrw that stopped reading and burning 12mths after purchase as the laser died ...
Like most things mechanical they will eventually go Poo it just depends on the build quality better quality components in the build generally lead a longer life than cheap nasty