It will come after you immediately after it becomes uncharmed.
Generally it's best to recharm that monster and have it fight again, though, because killing a monster that you've already charmed yields no EXP or items.
//Edit: Higher level beastmasters have a Calm ability that will make attacking monsters lose hate and go back to what they were doing.
Edited, Sun Jan 25 17:05:35 2004 by Einhander