Quote:
"As is the case with other optical storage media using phase-change technology there is a limit to the number of times the recording layer in a DVD-RW, DVD+RW and DVD-RAM disc can be reliably switched between its crystalline and amorphous states. It is estimated that a DVD-RW or DVD+RW disc can be rewritten approximately 1000 times and a DVD-RAM 100,000 times. In addition, these formats (under certain circumstances) employ defect management schemes to actively verify data and skip over or relocate problems to a spare area of the disc."
Source:
http://www.osta.org/technology/dvdqa/dvdqa11.htm
Even as you get close to the end of a disc's life you're unlikely to detect any worsening of sound or video quality. Digital technologies generally either work or they don't, with no middle ground.