You should conduct usability tests regularly to ensure that the user experience of your product is constantly improving. The duration will depend on the size and complexity of your project. Still, professionals generally recommend running tests at least once a month. It’s best to conduct this test every time a new feature or update is released.
Aside from that, usability testing sessions will be helpful if customer feedback indicates that certain features need further optimization. Make sure to find the right one in the usability testing tools market.