How long do you need to be working for the concil before you can ask for a secondment?

I work for the council as a library istant and i have seen a job as a librarian at a school. it is only fixed term till next july. i want to do it because i think it will be benifical to me and the library service. But i have only been working for the library service for 6 months. the librarian job is with the same council. Can i ask for a secondment? what do think the chances are of me getting one? anyone who can help i will be really gratefull