Hi,
They say replace chains at 1/16". However if you do that by the time
you get to your third chain at 1/16" it will work better than a new
chain with the worn rear cassette.
Rotating 3 chains with a cassette as it wears is the best way, and you
can take all three to 1/8" before needing new chains and new cassette.
Probably more, the jury is out on what stretch trashes front chainwheels.
rgds, sreten.