One should recite the three Quls (i.e. Surah Ikhlaas, Surah Falaq and Surah Naas) thrice every morning and evening.
عن عبد الله بن خبيب رضي الله عنه قال: خرجنا في ليلة مطيرة وظلمة شديدة نطلب رسول الله صلى الله عليه و سلم يصلي لنا قال فأدركته فقال قل فلم أقل شيئا ثم قال قل فلم أقل شيئا قال قل قلت ما أقول؟ قال قل هو الله أحد والمعوذتين حين تمسي وتصبح ثلاث مرات تكفيك من كل شيء (ترمذي رقم3575)
Hadhrat Abdullah ibn Khubaib رضي الله عنه reports that on one occasion during a rainy night we set out in search for Rasulullah صلى الله عليه و سلم so that he may lead us in Salaah. It was an intensely dark night. I then met Rasulullah صلى الله عليه و سلم and he told me, “Recite.” But (not knowing what to recite) I did not recite anything. He told me for the second time, “Recite.” But (not knowing what to recite) I did not recite anything. He told me for the third time, “Recite.” I then asked, “What should I recite, O Rasulullah صلى الله عليه و سلم?” Rasulullah صلى الله عليه و سلم replied, “Recite Surah Ikhlaas and Muawwazatain (i.e. Surah Falaq and Surah Naas) in the morning and evening thrice, it will suffice you as a protection from all harm.”
Source:Ihyaauddeen