As with LaserDiscs, there have been cases of "DVD Rot", where between the layers, the glue can corrode and the disc essentially becomes unplayable.
Out of hundreds, if not almost a couple of thousand, of discs, I've only had this once or twice. I know my "Kentucky Fried Movie" jammed after playing fine for years and just can't be read in any player now.
On average, and as with LD, the format <I>should</I> outlive us, but in practice it's anything but. A good, well-pressed DVD should live for 20 years + if taken care of and kept in its case. It could be as low as 5 years if the disc is not handled correctly.
That said, playing the same disc over and over shouldn't affect the life of the disc at all, since as Rand says, it's not being "played" as such, just being read. However, there is some link some have discussed between the heat generated by the player affecting the glue used in the layers, so a disc that continually gets played could just be getting hot and cold more often.
But you'd have to be playing that disc literally all the time...!?
Hope that helps a bit, but the safe option is to back-up your discs to a copy, or as I do, buy a couple of those titles that you know you're going to kick yourself over if it becomes unplayable.
