Does turning off Automatic Graphics Switching reduce battery life by much?

The discrete GPU uses much more power than the integrated one. Everything else being exactly equal, the difference between using the integrated GPU all the time and using the discrete GPU all the time is apt to be between two and three hours of battery life with a new battery. So turning off auto-switching will make a difference that could be that great if ALL the apps you use are ones that auto-invoke the discrete GPU, or it could be much smaller if most or all of your apps are ones that don't cause auto-switching when it's turned on.

No one can answer the question for you.

For a start, that thread discusses issues some users have with a blue screen after quitting apps, which may be related to switching graphics mode. However, the problem isn't universal so if you're not experiencing this problem there's no reason to disable graphics acceleration - you're applying a workaround for a problem that doesn't exist on your machine.

As for the general question of whether to use it or not, that's 100% up to you, based on the applications you run and how you use your machine.

If you're always plugged in, or never far from a plug, then you don't care if the accelerated graphics use more power since you're not going to run out of battery power anyway. Similarly, if you don't use any apps that use the discrete graphics then you'll stay with the integrated graphics anyway so it doesn't matter whether the switch is on or off. My point is that the decision of whether to enable it or not depends on you and your usage model, which no one here can know.

Personally, though, I'd leave it on all the time unless I'm planning to be powerless for an extended period (e.g. getting on a plane) in which case I might prefer the extra battery life.