Ki is the warrior's main asset. Once Ki is used, it can be regained 3 different ways.
One: a rejuvination tank, these will heal all hp and ki if a fighter is in one.
Two: the mystical senzu, these are very rare and found only on earth in one spot (or being carried around by warriors fortunate enough to have them), so its not really a way, but it is still a way to regain ki.
Three: after one full day of rest, half of the ki you have lost is regained, in another full day of rest it will all be regained. If you do any activity during these two days, for every hour you do something, its another 2 hours of rest needed.
*HP and LP Regeneration*
A character will regain HP at a constant rate of 10% per day. LP loss usually requires medical attention to properly heal. LPs are regained at the rate of 2 per day without treatmentanyway, but in a hospital, a character will regain LPs at a rate of 5 per day. LPs are only affected when a character has gone through all his HP, and if they lose all their LP, then plant them 6 feet deep, they're dead.