The answer depends on a few things that aren't obvious from your question.
What you mean by jointly held real estate matters a lot. There is a distinction between joint tenancy and tenancy in common, and that distinction comes into play if one party wants to force the issue in court.
What you mean by "partition" also matters. If you just mean cut up the real estate, that's relatively easy to accomplish. Getting the land surveyed is a likely first step, followed by negotiating a fair split, being mindful, of course, of local land use laws, and finally executing and recording deeds to finalize the transaction. Sounds like a month or two at most, realistically, to do it right.
If, by "partition", you are assuming a court fight in a partition action, then you could be looking at several months to a couple of years depending on the complexity of the case and the dynamics of the parties.
A good lawyer can help you avoid the potentially heavy litigation costs by facilitating a negotiated agreement to get the deal done in a way that's fair to both parties without going "all the way" in court.
Good luck with your situation. If you believe you may wish to hire a lawyer to help, please feel free to contact me. DISCLAIMER I am a lawyer, but I am not your lawyer unless you come to my office and we sign an engagement agreement. The answer provided here is for general purposes only, and does not presume to correctly address your particular circumstances. My answering this question does not create an attorney/client relationship.
If all co-owners agree, as long as it takes to create legal descriptions of the parts, which may require a survey; and to get whatever governmental approval that is necessary for the split. If you are talking a lawsuit, where the parties do not agree, it could take years.
If all agree - 48 hours.
If one has to sue the other... forever, because you can't partition jointly held property.
If the property is not jointly held, but held as tenants in common, then you CAN sue for partition and depending on the court, expect 1-2 years. Your mileage may vary. Also expect to pay a whole lotta dough to your attorney, appraisers, surveyors, hangers on and members of the entourage...
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