You'll know all of the bits have released when you run your spoon over the bottom of the pan and it feels smooth.
Most cooking cues are sight, sound, smell or taste. This one travels up a wooden spoon, which means it still works in a dark pan, under a sauce, with steam in your face, which is exactly where you're standing when you deglaze.
And it's a binary. Smooth or not smooth. There's no judgement call and no until it looks right.
The brown bits are the flavour
They aren't burnt-on debris. They are what happened where the protein met the metal, concentrated into a thin layer that's worth more than anything you could add to the pan afterwards. The word for them is fond, which is French for base or foundation, and that's what they're.
A splash of liquid lifts them because they dissolve in water, and that's why it takes both a liquid and a spoon. They are genuinely bonded to the metal, so the liquid does the dissolving and the spoon does the lifting, which is why the pan goes from rough to smooth while it happens rather than all at once.
So the pan bottom is the gauge. Drag under the spoon means flavour still stuck to the pan. Smooth means all of it's in the sauce now.
Two things worth knowing
On cast iron, deglaze with shallots rather than wine or vinegar. Acidic liquids strip the seasoning off the pan.
There's a second way to do this and it uses no liquid at all. Cut the heat, throw in the onion or the shallot, and wait a minute. Its own water comes out, pools instead of flashing off, and lifts the fond on its own. Nothing gets scraped, so there's nothing to feel under the spoon, and the cue above doesn't apply.