You should apply for ATAS as early as possible, as if you do not have your ATAS certificate you will not be able to make a valid Student Route visa application. An ATAS certificate is valid for 6 months from the approval date on the certificate. This means you can apply for the ATAS several months before you intend to make your visa application.
We recommend applying up to 9 months in advance to avoid any unnecessary delays or worries. The earlier you apply, the more likely that your application will be processed within the 20 working days guideline.