How can I measure the WiFi duty cycle / channel utilization in MikroTik routerboard RB922UAGS-5HPacD using Winbox?

Duct tape it to a spectrum analyzer?

Wireless snooper will provide this information but it depends on how you have your system configured: viewtopic.php?t=194728
It may not be possible to serve clients, either, so either a dedicated Mikrotik configured for this or, as others have said, move to a third party product.