State of grace

As observed by the Twelfth Doctor, the "state of grace" was the period before regeneration when a dying Time Lord could resist the changing process without consequence for a limited time where they were restored to full strength. During this period of time, the Time Lord would chose whether or not they would ultimately regenerate or they will grow weaker and eventually perish as time passed by. (TV: Twice Upon a Time) While a state of grace could be voluntary, some forms of death could induce a similar state unless they received aid. (TV: Planet of the Spiders, PROSE: Love and War)

The Doctor
The First Doctor, fearing the change, held off his own regeneration for several hours. However, unlike his future incarnations, the Doctor's face did not stay the same, becoming "mixed up" while in the state of grace. When the Doctor regenerated, he passed out as his state of grace ceased. (TV: Twice Upon a Time) However, his regeneration was comparatively peaceful than when his future selves regenerated after halting the process (TV: The Tenth Planet)

When the Third Doctor contracted radiation poisoning on Metebelis III, (TV: Planet of the Spiders) he spent ten years drifting in his current form in the TARDIS before he finally died upon returning to UNIT, (PROSE: Love and War) only regenerating with the aid of K'anpo Rimpoche. (TV: Planet of the Spiders)

The Fifth Doctor was able to hold off his regeneration from Spectrox toxaemia for several hours (AUDIO: Winter) while trying to retrieve the bats' milk needed for the antidote but finally accepted the need to regenerate when he was only able to acquire enough antidote for his companion, Peri. (TV: The Caves of Androzani)

The Ninth Doctor momentarily delayed his impending regeneration to explain what was happening to Rose Tyler. (TV: The Parting of the Ways)

After subjecting his body to a lethal amount of radiation, the Tenth Doctor delayed the regenerative process long enough to revisit each of his former companions from his own incarnation and his past selves as well (TV: Death of the Doctor) in a "final reward" before he ultimately regenerated, which severely damaged the TARDIS. A few minutes before the regeneration began, his "state of grace" ended causing the Doctor to experience excruciating pain, resulting in him collapsing to the ground, barely possessing the strength to reach his TARDIS. (TV: The End of Time)

When the Eleventh Doctor was granted a new regeneration cycle on Trenzalore, he was able to use the energy from the resulting "reset" to regress his body to a younger state and hold back the regenerative process for a time. (TV: The Time of the Doctor) During such, he phoned his personal future to ensure Clara Oswald stayed with his future self, (TV: Deep Breath) and later said goodbye to his companion before finally undergoing a full regeneration. (TV: The Time of the Doctor)

When severely injured by a Cyberman, the Twelfth Doctor was able to resist regeneration for several weeks despite struggling to walk. He further resisted it despite numerous Cybermen mortally wounding him, buying himself time to ignite Floor 0507. However, he struggled to hold it off when revived in the TARDIS, nearly regenerating completely. (TV: The Doctor Falls) The Twelfth Doctor encountered the First Doctor at this time, who was also in a state of grace, holding back his regeneration as he feared regenerating. Throughout this time, the Twelfth Doctor steadily grew weaker, nearly collapsing at one point while at Villengard and having to sit down for a few minutes to recover. After the adventure, both Doctors chose to regenerate. While holding his regeneration back a little longer to make an advisory speech to his future incarnation, the Twelfth Doctor continued to grow weaker and collapsed to the floor just before his regeneration occurred though he managed to stand up once more by leaning on the TARDIS console to help him to his feet. Similar to the Tenth Doctor's regeneration, the Twelfth Doctor regenerated explosively, causing the control room in the TARDIS to explode. (TV: Twice Upon a Time)