I have 2 CD-RW's and using them since 2001. They're from VERBATIM and on the back they guarantee that they can be written about 1000 times. That's what i know.
And by the way, isn't erasing those CD's something like "writing" on them again? ie re-ordering the magnetic structures to empty state. Guess that would reduce it's capacity to 500 real useful writes, or not?