There can be some local variation with this issue. Probation officers are the key people for this question. If they support early discharge, it will usually happen. If they are ambivalent, it might happen, upon a defense motion. If they strongly oppose it, it usually will not happen. So i'd suggest discussing it with the Probation Officer first. A problem that comes up with a request for early discharge from a five year probation is that most felony probation periods are longer than five years, so five years seems short to people working in the system, to begin with. It's a good idea to have a good, time sensitve reason or need for early discharge; and there better be a perfect or near perfect record as a probationer.
The answer to this question depends on the specific facts in your case. I suggest you talk with your probation officer, and then consult with an attorney regarding your specific situation. There is a lot more to the answer than just the crime you were convicted of and the amount of time you have served on probation thus far. Your criminal record, history as a probationer, the county you are in, etc., all play an important role in this.
I agree with my colleagues that it is hard guess whether a court would approve it. A good place to start would be with your probation officer. If the probation officer supports the request, you have a much better chance than if the probation officer thinks you should stay on paper.
This response does not create an attorney client relationship and is offered for informational purposes only. Only a lawyer fully versed on the facts and circumstances of your case can properly advise you on the case. I am licensed to practice in Minnesota, not every state. You should always consult with an attorney licensed in your area on how best to proceed.