You can let it drain all night. It won't hurt anything but don't expect to get another 21 ounces out of it unless you've got the engine tilted at an angle that raises the drain point above its normal location preventing proper drainage.
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Stated oil capacity may or may not be the same as the amount required for an oil change. For instance, I have a TDI VW. If I don't make the effort to get the oil out of the oil filter housing, there will be 8 to 10 ounces of oil left in there.
Best thing to do is let it drip until it quits, then finish up your oil and filter change. Take a note of how much oil was needed to refill the engine. Start the engine and let it warm up. Let it cool, then check the oil again. Top it off if needed. Take note of how many ounces it took. That will tell you what the 'normal' oil change quantity is. But NEVER just drain the oil and replace it with a stated sump capacity unless you already know that won't overfill the engine.
FWIW, a lot of engines can have their oil removed 'from the top,' so to speak by using a vacuum oil extractor, such as the Pella 6000 (that's the one I have). They work REALLY good in some applications, not so good in others. For instance, when I used it on daughter's Acura RSX, it gets every bit of the oil out. When I used it on a 3.0 V6 Ford Ranger, it leaves a quart or more.