Mr. Sullivan is correct. Most counties it is half. But you avoided prison on the charge and many judges will be hesitant to let you off early. I would suggest you try if half the period is over!
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There is no minimum. Generally it is one half, but if circumstances allow you can ask the Judge for early termination sooner, likewise, the Judge can make you wait longer. The decision is entirely in the hands of the Judge. A good presentation at a hearing on your request is what will make the difference, assuming you have completed every single requirement of your probation except for time.