How often do you replace your chain?

The more the chain is stretched and used in that state, the quicker the teeth will wear. I once bought a used bike that needed a new chain. I replaced the chain and found the cassette was pooched. So then I replaced the cassette and once that was grabbing hold well the chain rings couldn't handle it and I nearly ended up on the top tube the first ride out. In the end I had a new drivetrain.