Here's how it works:
* Monster Reborn is a Spell Card that lets you bring back a monster from your graveyard to the field.
* The monster you bring back stays on the field until it's destroyed by battle, card effects, or other means. It's not a temporary effect like a boost or a shield.
So, the monster summoned by Monster Reborn will stay on the field until it's removed, regardless of how many turns have passed.