I'm sorry to have to say that there is no way of knowing how long you will have to wait until you are allocated a start.

Several factors govern this, not the least of which will be monetary constraints, during a period when the Public Sector are making cuts, but it may just be something as simple as waiting until they have a sufficient number of recruits to start at the Prison Service College.

I would suggest however, that you get in touch with recruitment and see if they can give you any indication, you could be lucky, and find that they have been given approximate start dates.