Two mechanics (at different shops) told me that you should check for chain tension whenever you drain the oil and use that to determine when to replace the chain. Before the chain can hop a tooth (they rarely break outright, I'm told), it must have stretched enough to have slack beyond what the tensioner can take up.

Both suggested that I not change my chain as long as it's still tight (tightness can be checked with the cam covers off).

Could be wrong, but that's what I was told.