I have the Galaxy 4 and I don't recall an update ever taking that long. I wonder if it's caught in a loop. I'd say give it a few more minutes and if it keeps up, remove the battery count to 30 then replace it. That will do a hard reboot on it.
Unfortunately, updates are a necessary evil, so if it wants to update, try it again. If it gets stuck again, you may want to take it to you phone dealer for their opinion.

Hey, guys, my ancient phone finally kicked the bucket and I decided to take the plunge, sign on with a large evil phone corporation for two years, and get one of their smartphones. The idea I could read fanfiction anywhere I went was just too attractive! So this morning, I decided to mess around with it, to learn how to use it. It (A Samsung Galaxy 5) let me do a couple of things, like check my e-mail and navigate to Archive of Our Own on the internet, when, suddenly, it asked for permission to update, saying it would shut down for a short time... I can't remember how long. Every time I try to turn it back on, the main screen lights up, and at the top, it says that this is automatically updating, and then that, then the screen goes black again. It's been doing this for, like, 15 minutes now. Is that normal? Does it happen often? Any advice for the clueless one, here? Did I make a mistake by giving my permission for it to update?

Thanks for the very practical advice!