Is it more complete and preferable, Islamically, in the regular Sunnah prayers and the Duha prayer, to prolong them or to make them brief, and what has the Sunnah brought forth concerning that, and is supplicating during prostration in these prayers better, or is it better to reduce it and leave most of it for the last third of the night?

It is sunnah (recommended) to prolong the prayers in which this was reported from the Prophet, peace and blessings be upon him, such as the pre-Dhuhr sunnah prayers. It is sunnah to shorten prayers in which it was reported that he shortened them, such as the Fajr sunnah, the Duha sunnah, and the Maghrib sunnah. As for the 'Isha sunnah, nothing authentic has been reported specifically regarding prolonging or shortening it. Furthermore, nothing authentic has been established regarding the superiority of supplication during prostration over supplication in the last third of the night.