Are you applying to a LA or voluntary organisation. I'm guessing it's a LA. They should have been in touch with you before now, even if it was just to say they were not yet able to begin your assessment. LA social workers are usually very "snowed under" but more in child protection than adoption. Did they give you any indication at the prep group how long it would take to do assessments?
I am a retired sw and team manager for a Fostering & Adoption team, and we always kept applicants "in the loop" and made sure that they knew what was the next step, so I don't think it's good that you haven't heard anything, and this is almost certainly because of workloads. Even so I would expect them to give you an idea about timescales for starting your assessment. I'm afraid waiting can be the name of the game as far as fostering and adoption is concerned and I know things have changed a lot since I retired in 2009.
Why don't you give the social workers who ran the prep group a ring and ask if they have any idea when they can begin the assessment. Your application will be grinding through the system but it's not good practice to leave applicants in the dark and wondering what's happening.
I think there are some new timescales laid down by the govt for adoption, but many LAs are very short of social work staff and if this is the case in your LA, that will also cause a problem. Best thing to do is start phoning the sws as it could take several days to reach them, if they happen to be out, on leave/off sick, on a course etc etc. Think you will find you often have to give social workers a nudge.