Complex sentence: Peter stayed six years and learned a large chunk of the opera repertoire in a theatre with very high standards--the summers were nearly always spent working at Glyndebourne as assistant to his father and later doing his own productions and restaging his fathers.
Split into simpler sentences: Peter stayed six years and learned a large amount of the opera repertoire in a theatre with very high standards. However, the summers were nearly always spent working at Glyndebourne as assistant to his father and later doing his own productions and restaging his fathers.