It is recommended that a person should offer the five daily prayers at their respective prescribed times. That is to say, each prayer should be offered at its special time of significance (fadhilat). It is not sufficient to observe as much a gap of time after the prayers as required for a recommended prayer or a recommended supplication. The criterion, therefore, is only the time of significance.
The preferable time or the time of the significance of Zuhr prayers is from the commencement of the time of Zuhr prayers up to the time when the shadow of the indicator which appears after midday equals its length. The time of the significance of Asr prayers starts from the time when the shadow of the indicator equals its length till the moment when the shadow of the indicator doubles its length. The time of the significance of Maghrib is from sunset till the disappearance of the redness which usually appears in the West after the sunset. The time of the significance of Isha prayers is from the time when the said redness disappears till completion of one-third of the night. The time of the significance of Morning prayers is from the time when the whiteness rises from the east and continues till it becomes bright.
 - Bani Hashem Khomeini, Muhammad Hasan, Tauzihul Masail (Islamic Laws) of Maraje', vol.1, pg.420 – 421, 13th edition, Islamic Publications Office, Qom, 1996.