There are two basic philosophies to chain replacement. The dominant one these days is to replace the chain someplace in the 1/2 - 1% linear stretch range to prevent accelerated wear on sprockets. There's plenty of debate about how to measure, but a decent general consensus about the "safe" replacement range.

The other, older school, philosophy is to forget about trying to replace before killing the cassette, and to run both chain and cassette for about as long as you can. Of course, this may take the chainrings with it, but they're much more tolerant of wear than the smaller cassette sprockets. Running this way, you can probably go to about 2-3% stretch before things go sour.

Either way, you want to know the facts before you decide. My preferred method is the old school 12 measurement with a ruler against a taut chain.