Breeding involves combining two monsters to result in another monster. Only certain monsters are breedable, although unbreedable monsters are regularly offered or rewarded during breeding events. https://monsterlegends.fandom.com/wiki/Breeding
All legendary monsters have the same breeding time: 1 day 22 hours. This time is without any boost. If you have the guardian max level then it takes only 9 hours and 11 minutes.