The second ability doesn’t have a duration. It lasts until the creature leaves the battlefield.
2023-04-14 • wotc
If the target of the second ability stops being a creature after the ability has resolved, it will also stop being an Angel until it becomes a creature again, if applicable.
2023-04-14 • wotc
For the last ability, if a card in your graveyard has {X} in its mana cost, X is 0.