You should start the procedure as early as possible to ensure timely visa issuance. Typically, students can apply for a visa approximately 2-3 months before the start of their studies, upon receiving an official invitation from the university. In some cases, invitations may be received even earlier, but the application submission period allowed by the country may open only 4-6 months before the start of studies. In such cases, you'll need to wait for the visa application period to begin.
Please note that the consulate may extend the processing time without providing reasons, especially during peak submission periods or for additional review purposes.