What should I do with the remaining time after rising from rukūʿ in the night prayer, if the dhikr I know only takes half a minute, and the Prophet, peace and blessings be upon him, used to prolong this standing? Should I repeat the dhikr or repeat "To my Lord be praise"?

It is prescribed to repeat the saying "To my Lord be praise" (لربي الحمد) when rising from bowing (ruku'), as mentioned in the hadith of Hudhayfah (may Allah be pleased with him). It is also permissible to repeat other forms of remembrance (adhkar). When prolonging the bowing, prostration (sujud), standing after bowing, and sitting between the two prostrations, one can repeat the reported forms of remembrance, such as saying: "Our Lord, to You be praise, abundant, good, blessed praise, filling the heavens and the earth, and what is between them, and filling whatever You wish besides. You are worthy of praise and glory" (ربنا ولك الحمد حمداً كثيراً طيباً مباركاً فيه ملء السماوات والأرض وما بينهما، وملء ما شئت من شيء بعد، أهل الثناء والمجد), or supplicate with whatever one wishes, as mentioned by Sheikh Ibn 'Uthaymeen.