When you want her to. =P lol. Kidding, elder sims actually die when their time is up, which is technically unpredictable. You can increase it with the lifetime aspiration reward the Elixir of Life, which will reset the sim back to the beginning or close to the beginning of the elder stage. To kill her off, you can delete her using cheats or make her die using cheats. Deleting would result in her being removed from the household painlessly, while death would make her daughter, Othelia Specter, and her other son Nervous Subject, remember the event and generate a tombstone which she would haunt.