Mine will stick sometimes for weeks or maybe even months. I think it switches when they push out an update, it's totally unpredictable when it will happen. At least on iOS, it usually notifies me that it's switched so I know I need to switch it back.
I'm not aware of one. As @eyelidlessness pointed out in the comment above, it seems to change back randomly, so I think it would be hard to detect when it changes.