When is it correct to say: "O Allah, make us firm at the time of questioning," and is it [said] after the prayer or at any time?

It is permissible to supplicate for steadfastness during questioning, both in prayer and outside of it. However, supplication with this specific wording has not been reported in the Sunnah for use within prayer. What has been reported after the Tashahhud and before the Taslim is seeking refuge with Allah from the trials of life and death, and the trial of death has been interpreted as the trial of the grave. It is preferable for a Muslim to supplicate with the prayers that have been reported in the Qur'an and Sunnah.