The DVSA conducted a survey of students, who had just passed their driving test.  It is claimed that on average these people had an additional 20 hours of practice, in their own car or supervising driver car.  

This practice is used to support the regular lessons given by an ADI. 

The DVSA have issued guidance, to be read by supporting drivers, and the learner themselves.  It is recommended, you record your private practice to show your instructor.  The link below will take you to the DVSA information.