Methods for economic assessment, e.g., cost-benefit analysis, are often used in the rail sector to evaluate large infrastructure investments such as building new high-speed railway lines. With larger railway networks and aging infrastructure, these methods can also be used for maintenance planning decisions. In this paper, we focus on basic maintenance and the newly introduced concept of maintenance windows in Sweden. These are pre-allocated slots in the annual train timetable dedicated to performing, among others, periodic/frequent maintenance activities such as inspections, maintenance and repairs. To justify the pre-allocation of such windows, this study presents a method to find minimal utilization rates depending on window designs and traffic situations. Using a cost-benefit approach, the maintenance windows are assessed using a total social cost including maintenance work costs, loss in traffic production and reliability gains in future traffic. Based on a case study from the Southern main line in Sweden, we study the minimal utilization rate in different test scenarios, i.e., night or day shifts, asset degradation functions and designs of maintenance windows. The results show that lower utilization rates (5-50%) can be accepted during low-volume traffic or for partial closures, while higher utilization rates (50-90%) are required for full closures during high-volume traffic. Whether the rates are measured as share of used window time or share of utilized windows is less important, especially when higher utilization is required.