In the Nephalem Rift instances, players usually only play until the minion quota is met and the Rift Guardian is spawned and defeated. Afterwards, the player can talk to someone in town to initiate a countdown and closes the Rift. What happens if you never fill the minion quota and just continue running through the Rift? Or, say you kill the Rift Guardian but then continue on as opposed to returning to town and shutting the Rift? Will the Rifts go on forever or are there a set number of levels? Is there any reason or benefit to continue exploring a Rift once the Guardian is defeated?
The size can go up to 10 levels. They vary though so not every one goes to 10. I don't believe there is any specific benefit to continuing, however, you do get to keep farming without spending more rift tokens.
I'll note both these sources mention that you get a Horadric Cache after completion, however, I believe that was moved to completing bounties. If I find a more up to date source I will edit it in just to be thorough.
It's a finite number of levels, but if you're you're looking for legendaries it's definitely worth going on, because your legendary drop rate is +100% inside rifts!
I mean, if you're drowning in rift fragments you might just wanna start another one and start working toward another rift guardian, but if you only have a few, play all the way through and maximise your play time with the drop rate bonus.