You are eligible to withdraw from your retirement account once you turn 59.5. You can also start withdrawing funds before this age if you have a gap of over 10 years between the time money was deposited into the account and when it will be withdrawn, or if you're using up all of the funds in the account.
You can withdraw as much as you want from your retirement accounts without paying a penalty or tax. All of the money that has been deposited into the account must be withdrawn, and it does not matter if you take all at once or over time. The amount taken out will count as taxable income for the year in which it is withdrawn, but it will not be added to your taxable income for future years.