What are the times when supererogatory prayers (Nawafil) are recommended, compared to the times of obligatory prayers (Fara'id)?

The times when voluntary (nafl) prayers are recommended are all times, except for three: after Fajr prayer until the sun has risen a spear's length, after Asr prayer until the sun sets, and at the time of meridian. This is supported by the hadith of Amr ibn Absah regarding the prohibition of praying at these times because the sun rises and sets between the two horns of Satan, and disbelievers prostrate to it at that time. Also, Hell is stoked at the time of meridian.

As for specifying these times by the clock, it is difficult due to their variation with different countries and seasons. Sahar (pre-dawn) is just before Fajr or the last third of the night. The sun rising a spear's length occurs approximately fifteen minutes after its appearance. Zawal (meridian) is recognized by the beginning of the lengthening of the shadow after it has reached its shortest point.