The information about the _Plant of Everlasting Youth_ form the Sumerian mythos can be found on the second half of **The Epic of Gilgamesh**.
In the _Tablet eleven_ Utnapishtim's wife asks her husband to offer a parting gift to Gilgamesh, so he learns that
> at the bottom of the sea there lives a boxthorn-like plant that will make **him young again**
(Note: there is already a related question with an answer that states that this plant _was most probably a species of Rhamnus_ ).
The _Tablen eleven_ states that
> if you can possess this plant, you'll be again as you were in your youth [...] with it a man can regain his vigour
and that Gilgamesh was planing to test this plant on a man of old age to see it the plan would rejuvenate him.
The epic explains how Gilgamesh loses the plant, so he can never make use of it. It is not described if it rejuvenating effects ara permanent or just a work-only-once-per-take property.