Determining the amount of money to withdraw from retirement accounts is one of the biggest challenges retirees may face. This decision is complicated by the fact nobody knows how long they are going to need the money to last. For this reason and others, it is a good idea to plan for a longer period versus a shorter one and potentially make adjustments along the way.
Other complicating factors can also influence the decision such as market returns and the desire to potentially be more active in the early years of retirement.
There are many ways to approach the decision. The most important thing is to understand your options and then build a plan that suits your unique situation. For more information on the subject, read this How Much Can I Spend article where spending retirement savings is discussed with illustrations, a historical review and investment considerations.