Checked/adjusted when the valve lash and cam index is being checked annually.
When doing a high mileage rebuild and change both chains at the same time.
Might not be book correct, but thats how I see it anyway.
The tensioner has a fair amount of adjustment length to it, so unless the upper chain is really old, or you see problems like missing rollers, you can generally go to a rebuild as Darren says. These are not Volvos or BMWs, so you're unlikey to get 300,000 miles on an engine without tearing it down at least a couple times.
Do look for missing/broken rollers when you have the cam cover off, and look at the sprocket teeth for wear and/or misalignment, which happens, but is pretty unusual. I've seen a tensioner that somehow got in a little crooked and wore its teeth and the chain pretty badly. But that's not the norm.
Lower chain cannot realistically be changed without removing the front cover.