You'll know they're ready when they're light amber and the bubbling quiets.
Sight and sound in one test, and the sound half is the part nobody writes down.
Colour alone lies
Sugar and starch brown before the water is gone, so something can look exactly right and still be limp inside. Pulling on colour is how you get a shallot that's the correct shade and bends instead of snapping.
The bubbling is the water leaving
What you hear in hot oil is moisture boiling out of the food and forcing its way up through the fat. While it's loud, there's still water in there. When it quiets, the water is gone, and that's the same event as crisp.
So the sound is a direct read on the only variable that matters, and it's available with your back turned and your hands doing something else.
Only while the oil stays low
Two things are happening at once in that oil. The food is drying out, and it's browning. At around 325°F and under they finish together, which is why the sound going quiet and the colour arriving are the same moment.
Push the oil hotter and browning wins the race. The outside colours and the pan goes quiet while there's still water in the middle. Shallots fried that hot are crisp on the plate and soft by morning. That's the difference between a garnish you eat standing up and a jar that keeps a month.
If you want fried shallots to keep, keep the oil low enough that the bubbling doesn't stop until the middle is dry too. A pound of shallots takes ten to fifteen minutes, not five, and most of that is water leaving before anything starts to colour.
One more place it doesn't apply: anything dredged in flour, cornstarch or Wondra. The bubbling you hear is water leaving the coating, not the food inside it. The crust dries and goes quiet long before the food does, so a coated shallot can snap while it's hot and still go soft in the jar.
Before anything goes in
You should hear a sizzle. If not, take it back out and let the oil heat more. That belongs to shallow frying and pan work. It's not for a deep fry, where you aren't lifting food back out of three inches of hot oil to reheat it.
Air fryers don't count
Air frying. No oil, no bubbling, nothing to listen to. That's a different technique wearing the same word.