Duha Prayer Time
Question

What is the proper time to offer duha prayer which is also called Ishraq prayer or Chast Prayer?
Praise be to Allah.What is Duha prayer?
Ishraq prayer is Duha (chast) prayer offered at the beginning of its time. These are not two different prayers. It is called Ishraq because it is done immediately after sunrise (shuruq) when the sun has risen to a certain height.  
Shaykh Ibn Baz said: 
“Ishraq prayer is Duha prayer done at the beginning of its time.” Majmu’ Fatawa al-Shaykh Ibn Baz, 11/401 
Duha Prayer Time
The time of Duha prayer is from the time when the sun has risen to a certain height, until just before the time for Zhuhr prayer. 
Shaykh Ibn ‘Uthaymin defined it as being from a quarter of an hour after the sun has risen until ten minutes before Zhur prayer. (Al-Sharh al-Mumti’, 4/122)
So all of this period is the time for Duha prayer. 
The best time for Duha prayer
“It is better to pray Duha prayer after the sun’s heat has become intense, because the Prophet (peace and blessings of Allah be upon him) said: ‘It is preferable to offer Duha prayer when the sun’s heat has become so intense that even the young of the camels feel it.’” Narrated by Muslim, 748 (Ibn Baz: Majmu’ Fatawa, 11/395) 
The scholars defined this as being when one quarter of the day has passed, i.e., halfway between sunrise and Zhuhr prayer. (See Al-Majmu’ by al-Nawawi, 4/36, Al-Mawsu’ah al-Fiqhiyyah, 27/224)