Is the opening supplication (Du'a al-Istiftah) recited in prayer only at its very beginning, or every time one rises from prostration to the next unit of prayer (rak'ah), or every time a new obligatory (fard) or supererogatory (sunnah) prayer is started?

The opening supplication (Du'a al-Istiftah) is prescribed in the first rak'ah of the prayer after the takbirat al-ihram, whether the prayer is obligatory or supererogatory. Ibn al-Qayyim mentioned several formulas for Istiftah in "Zad al-Ma'ad," including: "O Allah, distance me from my sins...", and "I have turned my face towards Him who originated the heavens and the earth..." (and it is said that he used to say this during the night prayer), and "O Allah, Lord of Jibril, Mika'il, and Israfil...", and "O Allah, to You be praise, You are the light of the heavens and the earth...", and "Allahu Akbar, Allahu Akbar, Allahu Akbar, much praise be to Allah...", and "Allahu Akbar ten times, then glorify Him ten times...". All these types have been authentically reported from the Prophet (peace and blessings be upon him).