If the battle was not successful, there is no timeout and you can repeat the battle any time (as long as you fulfill all requirements again).
However, if you have defeated a notorious monster in a monster hunt battle, there is a timeout. This varies from 2-30 game days, depending on the specific monster you have fought. Most timeouts lasts for ~7 game days, some are shorter some are longer.
You can use the tavern and the camp fire to speed up the respawn.
Hope that helps.
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