If you are submitting a written option in Clinical Biochemistry or Immunology you can do so at any time after completing the FRCPath Part 1 exam, before or after you sit the FRCPath Part 2 exam.
For all other specialties requiring written options (i.e. Clinical Cytogenetics, Embryology, Forensic Pathology, Histocompatability & Immunogenetics, Molecular Genetics, Toxicology, and Virology) you must submit your project at least four months before the closing date of the FRCPath Part 2 exam session that you wish to enter. The closing dates are normally the first Friday of January for the spring session, and the first Friday of July for the autumn session. So projects are expected to be submitted by the end of March and September, respectively.