I know that if I record and re-record +RW disks on my DVD recorder they only last a short while ... I have just bought a DVD recorder with a hard drive ... I presume that when I watch recorded programmes and then delete them, over a lengthy period, will the quality remain the same? I seem to be thinking of the hard drive in the DVD the same as a worn out disc I think ! Hope that makes sense ...
in practice this would take so long that any concern about the drive wearing out can effectively be ignored. the drive will most likely last far far longer than the rest of the recorder.
You can extrapolate from the length of the warranty. WDC for example give a three year warranty on drives designed for AV applications. Experience says that 3 - 5 years is a pretty reasonable expectation, but I've known drives that were still going strong after 8 years.