Usually, more than one usability test is needed. To permit iterative usability testing, allow several months for user interface design. Allow enough design time prior to the first usability test so that the UI design can support at least several important tasks. In general, a usability test can be run, analyzed, and reported in three or four weeks, leaving aside the time required to construct the product simulations to be used in the test, but including the time required to recruit participants. Allow enough time between the tests for re-design of the user interface and construction of another UI simulation.
For a product that is already known to be highly usable and is being augmented with a limited additional feature set, two usability tests may suffice. For a more significant product release, plan to run at three or four usability tests to shape the UI design. Consider running more or larger-scale tests for product releases having any of these characteristics:

• Multiple, well-differentiated user roles
• More than the usual number of input modalities, e.g., touch screen available, or pointing device optional
• Multiple display formats, e.g., for both desktop and handheld UIs
• Requirements for especially efficient user interaction