Water Infrastructure Flexibility Act This bill amends the Federal Water Pollution Control Act (commonly known as the Clean Water Act) to allow municipalities to develop a plan that integrates wastewater and stormwater management. A permit for a municipal discharge under the national pollutant discharge elimination system that incorporates an integrated plan may integrate all requirements under the Act addressed in the plan. Those permits may include a schedule of compliance that allows actions for meeting water quality-based effluent limitations to be implemented over more than one permit term if the compliance schedules are authorized by state water quality standards. Those actions may include implementing green infrastructure as part of a water quality-based effluent limitation. (Green infrastructure includes measures that mimic natural processes to store, reuse, or reduce stormwater.) The bill establishes an Office of the Municipal Ombudsman in the Environmental Protection Agency (EPA) to provide: (1) technical assistance to municipalities seeking to comply with the Clean Water Act and the Safe Drinking Water Act, and (2) information to the EPA to ensure that agency policies are implemented by all EPA offices. The EPA must ensure that specified EPA offices promote the integration of green infrastructure into permitting programs, planning efforts, research, technical assistance, and funding guidance. The bill establishes requirements for revising the EPA's 1997 guidance about combined sewer overflows, including by setting forth criteria for determining the ability of households to pay utility bills. (Combined sewer systems collect rainwater, sewage, and industrial wastewater into one pipe. During storms, the combined wastewater sometimes exceeds the capacity of the treatment plant. When this occurs, combined sewer overflows discharge directly into water bodies.)