do a search on this there is loads. but to summerize, when the rails look like beer bottle colour (brown, if it is green you got bigger problems;-) )and/or the chain is too stretched (this depends on the model).
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)