Gift of immortality.

[Eldrazi Summoner]

Does it only work once or keep working over and over?

[Rass]

 {Gift of Immortality}

It helps to link the card in question. (Quote me or use the book in the upper right corner)

Yes it keeps working as long as it is enchanted.

Say it gets returned to the battle field and bolted again before it can be enchanted again then it will not come back.

[TreeTopSky]

Yes, as the card says: At the beginning of the next end step, attach that card to the creature.