Besides how long you expect to live after retirement, the level of income you want will play a major role in how much you need to save. Predicting your income needs at retirement are tough, but it gets easier the closer you get to retirement. Retirees usually don't have the same expenses as younger people. They may no longer have a mortgage, and they likely don't have to fund college education for kids. However, other expenses increase.
Retirees have higher health care costs.
Retirement income and years of retirement will dictate how much funds are required at retirement. The younger you are, the more challenging it will be to estimate these factors. It is better to have funds and not need them, then need funds and not have them.