This is a very basic requirement. See [Deprecated] Design Pattern: Motion Sensor Timer if you want to implement this yourself. If you’d like to just instantiate and configure something, see Debounce [4.0.0.0;4.9.9.9].
At a high level you’ll create an Item that is linked to the light. The Item linked to the motion sensor is added to a Debounce Group. That Item needs the Debounce metadata (see the link for details).
Properly configured, the light will turn ON immediately upon the first motion detection and OFF only when no motion has been seen for a given amount of time.