It depends on the bird itself. You gotta watch em a little.When you see them able to actually flap the wings and make it up to elevated perches, its time.
And you have to keep it up, every few months.
Even then, youll be amazed at how high they can get.
I clip both, too. The one wing clip sounds nice in theory but Ive seen it fail.
Regardless, whether you clip one or both, make sure there are no elevated perches anywhere near the fence or edge of whatever compound you use. Chickens are driven by their nature to "go UP," and any chance you give them to get over the fence will be squarely your responsibility.