I have about 40k km on my original chain, still looks good, the rear sprocket has not been swapped out yet either and looks fine. I have swapped out the front though, it was showing slight wear.
I am using a chain oiler which makes all the difference in longevity.
If your chain starts to stretch (needs frequent adjustments), or it starts making clunking noises (binding links), or the rear sprocket starts to hook and needs replacement, then you need to replace the chain.
A few guys have had their chains break on them, but it seems that is rare, the bummer is a broken chain at speed can take out the engine block requiring a new engine.