Probably never.  Bylaws normally don’t have anything to do with 501(c)(3) eligibility.  They are rules of governance of the organization within your state law structure.  Once the IRS accepts your original bylaws in your application for recognition of exemption—to be sure that they don’t allow you to engage in substantial non-charitable activities, direct you to provide private inurement to the insiders, or take some action that is otherwise specifically prohibited for charities—it is unlikely that you will enact a change that would disqualify you from charitable status.

It makes sense to review your bylaws periodically, every four or five years perhaps, to assure that they still reflect appropriate rules for governance and that you are following the rules you have established.  But if bylaws are written to provide flexibility and basic governance procedures, they should not need constant revision.  They should essentially be an “evergreen” document unless you make a fundamental change in the structure of the organization.  (See Ready Reference Page:  “Bylaws Function as ‘Constitution’ of Nonprofit Corporations”)