During the events of The Heroes of Olympus, Gaea opened up the Doors of Death so that the monsters could come back easier.
However, I could find no mention of how they got back without the Doors. But we know that they come back, as the Furies and the Minotaur regenerate.
And if there was an easy way to get out of Tartarus, it wouldn't be such a great prison, and we see that it is used as a prison for the Titans.
How do the monsters get out of Tartarus normally?