I've only used it a few times so far but it only takes about 10-12 seconds to get a reading. Open S Health, click the menu icon, click Sp02, and it will wait for you to touch your finger to the sensor. After about 10 seconds it shows your reading and records it. You can view daily, weekly, and monthly charts of your readings which is cool.
The widget does not seem to be customizable really so you can't create a direct shortcut to it but if you tap the widget it takes you into S Health so it's still only 2 tap away. In short, with the widget you can begin the reading with 3 taps + the finger touch to the sensor on the back middle. Definitely more convenient than carrying around a pulse oximeter / being a backup... assuming it's accurate enough. They of course say these things aren't for medical purposes officially.