Not an expert by any means, but given personal experience: By taste it feels fairly obvious when the roast is over 3-4 weeks old. And it's not only perceptible by taste, it is fairly difficult to ignore while blooming a pourover when it's fresh but specially so during calibration for espresso: when a grind is on its first week it's much harder to find the right spot, and when it's over ~3 you're clearly grinding noticeably finer.
Caveat: If you're unfamiliar with what "normal fresh" is for a specific bean of course it's going to be hard to tell at which stage it is but it doesn't mean that you wouldn't be able to state a clear preference if presented with the same bean before, during, and after its ideal degassing window.
And double caveat: A bean is not a bean is not a bean. Dark roasts degas much faster than light roasts and there's some wild variation out there in what's grown.
Caveat: If you're unfamiliar with what "normal fresh" is for a specific bean of course it's going to be hard to tell at which stage it is but it doesn't mean that you wouldn't be able to state a clear preference if presented with the same bean before, during, and after its ideal degassing window.
And double caveat: A bean is not a bean is not a bean. Dark roasts degas much faster than light roasts and there's some wild variation out there in what's grown.