Decision Ready Checklist and 885 Application

Hello,

I recently applied for my 885 visa application and my agent attached a decision ready checklist to it since all my documents were in order. 
When i asked about the processing time, i was told that it would take around 2 to 3 months.  However, when i checked on the DIAC website, it says up to 18 months. 
I would like to get an external opinion if possible of what on average the processing time should be if all the documents are in order and were submitted promptly.

Thanks in advance,
Kevin