When you are personally lean enough to get a "good response" from a carb refeed.
Meaning -- you now have a level of leanness that has increased your insulin sensitivity via upregulating your insulin receptors, so when you introduce carbs in abundance, you get temperature increase, cosmetic muscle fullness, essentially an INSTANT response.
That could be different for everyone as some guys could get to that point at 300g carbs a day from high activity, while others will need to be sucked down into 50-150g a day for prolong period of time.
I am getting leaner off 600g carbs precontest, but damn sure do not call for a "refeed" as everyday is feeling like one....
Perhaps when I reach my PERSONAL "depletion level" which could be 250-350g carbs I will require a very high day....
-FF