In the world of bread there are two main periods of dough rising. There’s the period right after the dough is mixed, which is usually just called “rising”, though some fancy-pants baking types call it “primary fermentation” or “bulk fermentation”. “Proofing” is the period of rising that happens after your dough is shaped into a loaf, right before it’s put in the oven (those same self-serious bakers call this interval “secondary fermentation”).
Judging the right amount of proofing time for a shaped loaf is a bit harder than judging rising time. Whereas you can visually inspect dough that’s rising by cutting into it and looking for bubbles, you don’t want to cut into a proofing loaf until right before you bake (i.e. when you score it). Well-proofed loaves look puffy, yet the true test is a good poke with your index finger. Does it feel firm like a sausage? Then let it rise some more. Does it feel soft yet still a little rubbery and resilient? Then it’s time to bake it. Does it feel like a flabby gas bag? Well then, hurry up and bake, but cut your proofing time down considerably next time.
Home bakers get into a lot of trouble assuming that the skin of a proofing loaf is like a plastic bag (it tempts one to let the proofing go on and on in hopes of greater volume and bigger bubbles). In fact it’s a porous membrane that lets quite a lot of carbon dioxide gas pass right through it. The only thing that keeps the loaf from sagging like a stretched out balloon is that there’s plenty more CO2 where that came from. At least, there is as long as the food holds out.
Imagine your proofing loaf like one of those fan-driven dancing men you see down at the car dealership. Being made of hollow tubes of vinyl cloth, the tuxedo guy loses massive volumes of air as he dances around in the breeze. But as long as there’s plenty more air being pumped in, it doesn’t matter how much air he loses, he’ll still stay up. A proofing loaf is kinda the same way. As long as there’s more gas being produced than what’s escaping (which means the yeast are still feeding), it’ll keep rising. When the gas that’s escaping exceeds what’s being made, it’ll sag. The perfect baking point is right when the two forces are equal.
Honestly, you can never be totally sure when that point is, but with experience you can make a reasonable guess. Poke, poke, poke.