"The point of incarnation is to work out and purge the karma in a soul." -- Tibetian Lama
Since karma is infinite, having accumulated words thoughts and deeds since the infinite past, by the reasoning of Tibetian Buddhism, it will take an infinite amount of future lifetimes to change one's karma in order to become a Buddha. The Lotus Sutra teaches that a one to one change of thought, word, and deed is not only unnecessary but a foolhardy approach that ignores the reality of the present moment encompassing both the infinite past and the infinite future. Nichiren Daishonin teaches,
“If you want to understand the causes that existed in the past, look at the results as they are manifested in the present. And if you want to understand what results will be manifested in the future, look at the causes that exist in the present.”
"When we come to the essential teaching of the Lotus Sutra, then the belief that Shakyamuni first obtained Buddhahood during his present lifetime is demolished, and the effects of the four teachings are likewise demolished. When the effects of the four teachings are demolished, the causes of the four teachings are likewise demolished. Thus the cause and effect of the Ten Worlds as expounded in the earlier sutras and the theoretical teaching of the Lotus Sutra are wiped out, and the cause and effect of the Ten Worlds in the essential teaching are revealed. This is the doctrine of original cause and original effect. It reveals that the nine worlds are all present in beginningless Buddhahood and that Buddhahood is inherent in the beginningless nine worlds. This is the true mutual possession of the Ten Worlds, the true hundred worlds and thousand factors, the true three thousand realms in a single moment of life."
There is no need to spend lifetime after lifetime "incarnating in order to purge the karma in a soul." Depending on the teaching or Law [powerful or weak] and teacher [fully or partially enlightened], the faith of the practitioner [shallow or profound] and practice [powerful or weak], one can manifest supreme enlightenment at this very moment. The repeated, ever more intense efforts to reveal one's innate Buddhahood [while suppressing one's innate delusion] by chanting Namu Myoho renge kyo, creates the karma to quickly attain Buddhaood. If the teachings are very powerful [Namu Myoho renge kyo or Devotion to the Wonderful Law of the Lotus Flower Sutra] and the teacher[s] very wise [the Eternal Shakyamuni Buddha of the 16th Chapter of the Lotus Sutra and Nichiren Daishonin], the practitioner will never fail to attain enlightenment in this very lifetime.