It is not uncommon for the top surface of a microwave to become warm or even hot during extended cooking times, especially if you are heating something for more than 10 minutes. This is because the microwave's internal components generate heat during operation, and some of that heat can be transferred to the exterior of the microwave.
Microwave ovens are designed with heat dissipation mechanisms, such as vents and fans, to regulate the internal temperature and prevent overheating. However, during prolonged cooking or high-power operations, it is normal for some heat to escape and cause the external surfaces, including the top, to warm up.
While it is generally normal for the top of the microwave to get warm, it should not become excessively hot or reach temperatures that could pose a safety risk. If you notice that the top surface becomes extremely hot to the touch or if you observe any unusual smells, sounds, or other signs of malfunction, it would be wise to have the microwave inspected by a qualified technician to ensure it is operating safely.
Additionally, make sure that the microwave is placed on a stable and heat-resistant surface, and that there is sufficient space around it for proper ventilation. Blocking the vents or using the microwave in an enclosed space can lead to overheating and should be avoided.
Always follow the manufacturer's guidelines and instructions for safe usage of your specific microwave model.