You should conduct mobile usability test at various stages of the mobile app development lifecycle, including:

1. Early Design Phase: Conduct usability tests during the early design phase to gather feedback on wireframes, prototypes, or initial designs. This helps identify usability issues and refine the user interface before development begins.

2. Mid-Development: Perform usability tests as the development progresses to validate design decisions, assess the usability of implemented features, and identify any usability issues introduced during development.

3. Before Launch: Conduct usability tests shortly before the app's launch to ensure that it meets user expectations, is intuitive to use, and provides a satisfactory user experience across different devices and screen sizes.

4. After Updates or Changes: Whenever significant updates or changes are made to the app, conduct usability tests to evaluate the impact on usability and identify any new usability issues introduced.

5. Ongoing Basis: Usability testing should be an ongoing process, conducted regularly even after the app is launched. This helps ensure that the app continues to meet user needs, addresses emerging usability issues, and remains competitive in the market.

By conducting mobile usability tests at these key stages, you can gather valuable feedback, identify usability issues early, and iteratively improve the user experience of your mobile app.