Genetic deterioration of clones after generations?

Hi, what a great farking community!

I've got a newb question that I was surprised to not see addressed in the FAQs or by following the links in the tags. Maybe it's there somewhere, but I couldn't find it.

I heard from someone that over successive generations, the clones you make give weaker and weaker plants until you're forced to grow from seed again and start afresh. Can someone confirm or refute this, and tell me when I should expect this deterioration to occur?

Thanks a lot,
-Bloomer