When they are brought back they should be at the beginning of their life stage. As long as something doesn't kill them, they should live their full life stage.
Short would be 5 plus days.
Normal would be 10 plus days.
Long would be 40 plus days.
https://sims.fandom.com/wiki/Life_stage
~~Edit~~
It seems that I gave a bit of wrong information. After further looking into this if an elder is brought back by the grim reaper they will only get one more day to live and then they will die again.
https://ultimatesimsguides.com/sims-4-elders/