As far as I'm informed, you could just repeat a move until you get the desired effect, since there is no cooldown or mana. Are there limits to repeating a move, and if so what are they?
You seem to have misunderstood one of the most fundamental rules of Dungeon World:
You don't make the moves
Moves just happen when their trigger is met. That means you cannot tell your GM that you wish to, for example, take the "Defend" move. You have to narrate what your character does. If that triggers a move, then the move happens.
For example, if you narrate your character slashing at the goblin with their sword, the Hack and Slash move happens, because that move is triggered by attacking enemies in melee. However, the goblin might try to grab onto your character's back and sink their filthy fangs into your neck in retaliation. You can't just declare you want to Hack and Slash at the goblin again (well, you can, but it's going to require some creative narration to explain how you can use your sword against a foe in such an awkward place). Instead you might need to shake off the goblin, which might amount to Defying danger.
The circumstances may also prevent you from making certain moves: for example, if your party is fighting a dragon with scales thick enough to withstand all your blows, the GM and other players around the table might decide that your attacks don't count as Hack and Slash. You might be required to, for example, find a weak spot first - an operation that will most likely require plenty of other moves like Spouting Lore about the dragons' weaknesses and Defying Danger to dodge the dragon's attacks long enough to get to the softer spot.
The moves cut both ways
When a move calls for a roll and your result is 6 or lower, it means trouble. Sometimes the move directly tells you what the effect is, and otherwise the GM is free to make a move of their own against your character. For example, with a squirming and biting goblin on your back, failing to Defy danger could result in the GM deciding to Deal damage to your character, or Separate them from the party by stumbling down a chasm trying to shake off the goblin. Failing to Spout lore could result in Using up the party's resources (in this case, time spent observing the dragon in hopes of a weak spot) or an Unwelcome truth being revealed - for example that the dragon's weak spot is actually in a very dangerous place, like the inside of its mouth.
Because these moves that result from failure tend to necessarily involve changing the circumstances, you won't be able to indefinitely repeat a move in the same circumstance — quite soon, something is going to change to push the narrative forward.
Like pretty much everything in the game, it depends on the fiction.
For example, picking a lock. What things might stop you trying over and over again until you succeed? Perhaps you only have a short amount of time between guard patrols. Perhaps the chamber you are in is filling with water and you have to open the lock before you drown. Perhaps you have to open the lock before something chasing you catches up. Perhaps a failed attempt breaks your lockpicks.
If there is no time or resource constraint, then nothing stops you. In this case, no move is triggered. "The lock is open, what do you do now?"