How long can I be held in turkey

What you describe is usually not “detention” in the prison sense. In Turkey, this is typically a judicial control measure, namely a ban on leaving the country, under Article 109 of the Criminal Procedure Code. In other words, a person may remain free, keep their passport physically, and still be legally prevented from exiting Turkey if the court order is active.
There is no single universal maximum for every case. If the case is not within the assize court’s jurisdiction, judicial control can last up to 2 years, and in mandatory cases it can be extended by 1 more year. If the case falls within the assize court’s jurisdiction, it can last up to 3 years, and extensions can reach a further 3 years, or 4 further years for certain especially serious offences. The court must also review whether the measure should continue at intervals of no more than 4 months.
You are also entitled to ask the court to lift, reduce, or change the measure. Under Article 111, upon the suspect’s or defendant’s request, the judge or court may rule within 5 days after obtaining the prosecutor’s opinion, and decisions on judicial control can be challenged.
So the practical answer is this: they cannot keep you under a travel ban indefinitely without legal basis and periodic review, but they can keep it in force for a long time if the case category and procedural stage allow it. The real answer depends on the exact offence, whether the file is in investigation or trial, which court is handling it, and the date of the latest review order.