I just had my application approved and it was just short of 2 months from start to finish. I did most of the leg-work in Miraflores but had to visit Lima central Immigraciones office, to sort out an issue with the web-based appointments system. The form to use was Formulario F-004 (Cambio de Calidad Migratoria Residente Familiar De Residente).
That and making payments to the Banco de la Nacion took most of the back and forward efforts, apart from the Interpol 0.5 day trawl, for the Police Report. After everything was handed-in (copies of passport, marriage certificate copy, copy of last entry page on passport, DNI of my wife, a utility bill from our address) it became a waiting-game, occasionally logging-in to the immigraciones website to monitor the progress.
It's worth mentioning that Christmas/New Year would have probably caused additional delays in processing the application.
Hope that helps!