Pretty much this, when they've failed I send them for a rebuild! My last one from BG Motorsport lasted 30,000 miles ish! Some guys have only managed 8,000 ,depends on condition and how you drive I think!
And what are symptoms of failed dampers?
Mine have around 18641 miles and before I sold my 2004 I didn't feel anything wrong with them
I installed them on april 2009 and uninstalled on may 2013
I used my car to go on track by motorway and not very often to go to work