How long before I start volunteering as PRI interviewer after my first training?

I completed my one day workshop for PRI interviewers back in September, I then registered my details and availability on the Review Process System (RPS) and i have been checking from time time and so far i have had zero invitations to join interviews as an observer. Is it just because of the number of PRIs not being high enough currently to present the opportunity for my further training, or have i missed to do something else to proceed?