As long as they aren't dead* I will.
The next game they get used in just continues their story.
Of course the game also has to support them. For example;
*My 1/2ling Warlock, Bree - consecutively she's been an NPC, a PC through about 3/4 of CoS, a PC in several short-lived PBP efforts, made a few NPC cameos in two other campaigns I've run
(the campaigns are all different, just occurring at the same world/time game-wise. The players simply went to where she was at that time....}, made a very brief cameo as a 2nd PC in a Desert of Desolation campaign I was in, and is currently being used as a PC in a PF AP
{stated out nearly identical but as a Summoner - I'm missing 1! spell} that fills in the story gap between her exit from Barovia & her surprise appearance the end of the Desolation campaign.
After the PF AP wraps, though I love playing her, I envision retiring her for a while. But, to steal & paraphrase a line from the early Marvel movies,
"Bree Burrfoot will return...."
Whenever that is though she'll just be a few years older & the game & story will have to support a 9th lv 1/2ling Feylock. If it doesn't, then she stays in the file.
*My 18th lv Chellaxin Chevelier/Hellknight with 8 lvs of Mythic Power from our one PF campaign though....
It'd be pretty hard to work him into something. Sure, he's alive. But when the campaign wrapped he (and a decent sized army) was last seen heading south to Chelliax to wage war on the devil worshiping nobility wich controls his homeland. And after that he intends to take the fight right to Asmodeus's door step - in Hell - and end the devil.
I don't really expect to ever run him again. But if a DM would like to run that specific quest line for me.
* Remember up top where I said death was the limiting factor in bring back a character? Well, I've even got one for whom death needn't be the end.
I have a 5e Warforged Eldritch Knight, MK.7, who was made by some gnomes (his retainers from the Nobel background) & was being field tested when they got embroiled in the Tyrrany of Dragons adventure.
In what turned out to be our last session (the group fell apart) Mk.7 got his head crushed flat (killed).
But his gnome retainers survived!
They hauled him off to a lab/workshop & began repairing him. The plan was to bring in Mk.8 - same mini, different helmet..... But the game fell apart & that was that.
Mk.8 will make his entrance one of these days....
So yeah, sometimes I'll continue using a character from a previous game.