After (exactly) 5-months, my cellular data usage logs reset to zero. What happened?
Go to Settings > Cellular and keep swiping up until you get to the very bottom of the page. You'll notice a blue Reset Statistics option. Below this check the Last Reset date to see if it coincides with when your data went to zero.


If it does, you have your answer. It's likely you've accidentally tapped on Reset Statistics, or that perhaps someone else did.
If it doesn't, then it's impossible for anyone here to give you a definitive answer as to what happened.


As for how it's supposed to work, there's no time limit as such. One of my devices shows it was last reset on 19 September 2013!