Taxes during retirement

I have been searching for this information, but have not been able to find an exact answer. My mom will be retiring next year. She will have Social Security benefits, a pension and a 401(k) account. It looks like the first 10 K of Social Security benefit is not taxed. In the hypothetical situation of getting 22k in SS and 15k pension, how much would she pay in taxes for her 401(k) withdrawals? Is it better to withdraw some amount annually or keep all of the 401(k) funds in the account? Based on our calculations my mom should be able to live comfortably off of her pension and Social Security benefits(she’s very frugal with very low bills). anything I am missing when thinking about this? Thanks in advance for any information.