Theoretically, this process (by which an adult can revert to polyp form through transdifferentiation of its cells) can go on indefinitely, effectively rendering the jellyfish biologically immortal, although in practice individuals can still die. In nature, most Turritopsis dohrnii are likely to succumb to predation or disease in the medusa stage without reverting to the polyp form.
If the above were not true, the oceans would be totally overcome with them, because they still reproduce.
Please note: "can still die" means NOT immortal.