A. For a high level of confidence that you can maintain your lifestyle in retirement, you should aim to save at least 15% of your pre-tax income a year over the course of your working life.
This may seem like a lot, but it includes all retirement savings across different accounts plus any employer contributions. Of course, you may not be able to do this every year, but there are always ways to catch up along the way.
For a more detailed discussion, please read the Fidelity article here.