It appears every few times I open the app and the usual android back button doesn't make it go away, and there is no cancel button. What you have to do is toggle the toggle, then the button will change to "do not back up" and then you can press it. It annoys me how easy this makes it to accidentally turn on backup.
I'm always conflicted on this kind of thing, because for every person annoyed that it's asking there's a bunch that are complaining that they lost all their photos because their phone was stolen or something like that, and had no idea that they need to backup their stuff.
Having a notice that doesn't occupy the whole screen and can be dismissed temporarily or permanently is a much better and less predatory solution to this problem. This isn't looking out for the end user, this is urging them into a system where the host can profit off of the user's data, that also happens to provide a benefit to the user.
That's too bad for them, but at a certain point, people should be expected to have some level of understanding of the technology they are using. Giving people the information via a highly visible yet unobtrusive notification is more than enough to guide people that don't understand. If they decide not to follow the clear instructions then that's on them.
It's kind of like no child left behind. In theory, it's a good idea to make sure everyone is in a good position for success, but it's not good in execution because it negatively impacts more than it helps.
I didn't realize that I had turned it on on my prior phone. Welp, my phone absolutely died one day and would no longer power on. I was really sad about losing my photos and videos, but then when I got my new phone I was happy to find out that at some point I turned on photos cloud backups.