The regeneration cycle was the time during which the process of regeneration occurred. Every time a Time Lord regenerated into a new incarnation, it would take fifteen hours for the process to be finished. During this time there are moments of disorientation as the Time Lord heals.
Time Lords had a regeneration cycle consisting of thirteen lives. Engin observed during the Master's attempt to use the assassination of the Lord President after his ally Goth had died, that after the twelfth regeneration "there was no plan that could postpone death," when his own attempt to restore his ability to regenerate. (TV: The Deadly Assassin)
The newly regenerated Tenth Doctor credited the manifestation of a new hand to replace the one severed by Fadros Pallujikaa to having still been within the first fifteen hours of his regeneration cycle, which provided him with "just enough" residual cellular energy to regrow his severed limb. (TV: The Christmas Invasion)