I am DM in a game where one player likes to use the intimidation skill and menacing attack. I can't find anywhere that tells me how to get rid of the frighted condition so every time he uses either of those, the monsters are basically useless because they can't move up or land an attack forever. So, how long does the frightened condition last?
The specific source will list its specific duration.
For your specific case, Menacing Attack says how long it lasts - "On a failed save, it is frightened of you until the end of your next turn." - so it automatically ends at the end of the player's next turn.
More generally, effects that cause Fear allow subsequent saving throws to end the effect early and it automatically ends after a listed duration.
To address the problem of the player always frightening combatants.
As the DM, you mitigate the chance of success and or whether a roll is required based on the circumstances in the battle. And, you decide the effect a successful intimidation may have.
In one of my campaigns, the party was ambushed by a band of hobgoblin bandits lead by a chieftain. The character tried to intimidate the hobgoblins into fleeing. So, I had him roll his intimidation vs the chieftain's intimidation. On the fly I gave the Chieftain +3 on intimidation to represent his control over his band.
The character won the first round, and the hobgoblins backed up a step until the chieftain smacked them in the heads. The second round the chieftain won and the hobgoblins charged. When the chieftain and 3/4 of the bandits were dead, I gave the player an automatic success on his third intimidation check.
The players enjoyed the encounter because it gave a little more suspense than a flat roll that imposed the frightened condition, and everyone fled.
Duration of the frightened condition:
As VahallaGH mentions, the duration of the frightened condition depends on the source of the condition. The duration is usually mentioned in the description. Additionally, if the condition lasts more than one round, there is usually a saving throw each round to end it early.
As the DM you can also rule other things that break the condition; a melee attack for example. Fear for your life is generally greater than the fear of a person.