When a lightsaber gets low on power, what happens?
Does it slowly power down over time, or would it just turn off?
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A lightsaber running out of power was depicted in chapter 2 of the novel Shatterpoint (which is Legends, but you indicated that was OK in a comment). It happens to Mace Windu, after an authority had confiscated his lightsaber and secretly drained the power cell before giving it back to him. Here's how it's described:
While he was trying to feel his way toward what they might try next, he noticed that his blade cast a peculiarly pale light. Much too pale.
Even as he crouched there, staring drop-jawed into the paling shaft, it faded, flickered, and winked out.
He thought: And this trouble I'm in just might be serious.
His lightsaber was out of charge.
They use force powered crystals which, as far as I understand, only stop working if they are damaged, turned off via the force, disabled, or come into contact with the water (unless the jedi/sith/gray jedi is using a waterproof lightsaber. In these cases, the crystals stop providing the lights aber with power so the blade simply goes out. It's worth noting that the old lightsabers (pre-Old republic) required special packs attached to the jedi's utility belt to power them, but this problem is solved with "modern" lightsabers.
I don't recall an instance of a saber specifically being drained 100% from use in G/Disney canon, but we have specific mention of the lightsaber basically "burning out" because Obi-Wan forgot to turn it off (unclear what the water itself does), and needing "recharging", possibly on its own.
From The Phantom Menace
OBI-WAN : Sorry, Master, the water fried my weapon.
OBI-WAN pulls out his burnt laser sword handle. QUI-GON inspects it, as JAR JAR pulls himself out of the mud.
QUI-GON : You forgot to turn your power off again, didn't you?
OBI-WAN nods sheeplishly.
QUI-GON : (cont'd) It won't take long to recharge, but this is a lesson I hope you've learned, my young Padawan.
Assuming that the lightsaber blade requires full power to maintain a full length blade it would be reasonable to assume that the blade would shorten rather than dim as if it dimmed it would just disperse. This assumption is also made with the fan theory that lightsabers are beams of plasma is true.
I suspect the lightsaber blade shortens occasionally as it loses power. In this youtube video of Obi Wan versus Darth Vader, you can see Obi Wan's blade briefly shortens around 45 to 55 seconds. That's right before Vader says "Your power are weak old man." And Obi Wan's blade goes back up to full length before it shortens again at 60 to 65 seconds.
Although there is no official movie canon scene where a lightsaber sputters out completely, I think this snippet shows what happens on low power.