How long does it take to deflate a tube for a 26" bike tire?

I wouldn't bother removing the valve core - assuming its a schrader/AV/car valve, press something pointy into it for ~20 seconds and most of the air will come out.

If you want to roll the tube up, it is convenient to have a deflater tool, essentially a valve cap with a pin that presses the valve's stem and lets the air out. Convenient but not necessary.

If you have a presta valve then pressing the stem with your finger works fine too.

Why do you want to deflate it? Is the tire mounted on the rim and you want to remove it, or is the tube out and you want to get all the air out so you can fold/roll it up?

If you're removing a tire from the rim (or removing a tube from inside the mounted tire) you do not need to get all the air out. Press the valve pin and squeeze the tire several times to push the air out. This will take may be 60 seconds.

If the tube is out and you want to roll it up, press the valve pin (or remove the core, lay the tube out on a flat surface with the valve at one end, and start rolling from the other end. Will take 2-3 minutes.

If removing the valve, the air should be released within a few seconds.

If you have a schrader valve, in the center of the opening there's a pin that you press in to release air. It shouldn't take more than a few seconds to release all the air. If the tube is still on the rim, I usually sandwich the wheel between myself and a wall to squeeze a little more out once the air stops coming out on it's own.