If I am unable to solve a given Sudoku puzzle, how can I tell if or when I have tried hard enough and long enough to stave off dementia forever?
This is a very provocative question. I think you should disconnect your sense of mental well being from the success or failure of a given puzzle. That is, you should not judge your memory's capacity by any one puzzle left unsolved. I know that makes it difficult to tell the parameters for reaching mental fortitude, but I will encourage you to keep on doing Sudoku because even though there are no guarantees, it still keeps your brain active and builds mental capacity.