Part of your Magic Band should last forever and that's the touch points where you tap (near field tech) such as turnstyles, FP's, purchasing, etc.
The other part (far field tech ) for ride photos/videos etc will only last as long as the battery. Unfortunately there is no way of knowing about that part unless your ride photos do not show up on your
MDE account. Disney has said that they should last 2 years and that's why they would send them out with every on property trip booked. Quite a few here have reported that theirs have lasted longer then the 2 years and a few have reported that theirs did not last the two years.
It's too bad they didn't have a screen or something where you could test out your old bands for the far field stuff.