That is an incorrect translation. There is nothing in this world that lasts forever.
Only Nibbana, which is an asankata, lasts forever.
Kammic energies are sankata, created by the mind, via (abhi)sankhara.
Kammic energies are created by 7 javana citta. Javana citta start with “low intensity” in the first javana, and gain strength to about the fourth javana and then start decreasing in strength. So, by the time the seventh javana comes, it is weak. (imagine a sprinter starting from zero speed, gaining speed and then slowing down and coming to a stop).
So, kamma done with first javana citta are weak and can bring vipaka only during this life.
– Kamma done with the last javana is also weak and cannot bring vipaka beyond the next existence.
– The middle five javana are strong and can bring vipaka for a long time. I don’t think a specific time is given when that energy runs out. It probably varies for each citta vithi; some are more powerful than others.
However, there is a sutta where the Buddha says that when one does an alms giving (dana), that good kamma vipaka can last for 91 maha kappas. A maha kappa is about 10 billion years, so it is like a trillion years. That is a long time, but not forever.