Generally, it is advisable to register the First Information Report (FIR) at the earliest within 24 hours. If you are filing a complaint after a considerable time, there shall be a valid reason for it otherwise it can be used by the accused for his/her advantage. At present there is no period of limitation for criminal prosecution and a court cannot throw out a complaint or a police report solely on the ground of delay although inordinate delay may be a ground for entertaining doubts about the truth of the prosecution story.
However, if the police are not registering the FIR, then you can approach the magistrate under Section 156(3) of the Code of Criminal Procedure, 1973. A Judicial Magistrate of first class or of second class especially empowered under Section 190 of Code of Criminal Procedure can take cognizance of a complaint.
After the Magistrate takes the cognizance of a complaint, he examines the complaint as per Section 200 of Code of Criminal Procedure, 1973 by examining the facts and the witnesses. If the Magistrate finds that the complaint is with merits, then the case is deemed to be committed for a trial and the process is issued by the Magistrate
Reference: Section 156 (3), 190, and 200 of the Code of Criminal Procedure, 1973