Dear Sir,
The purpose of the Audit under Sec. 65 is to verify the correctness of turnover declared, taxes paid, refund claimed, and input tax credit availed, and to assess the compliance with the provisions of the CGST Act. It can be understood from this provision that the audit is to be initiated by the Department after the due date for filing the returns. The due date for filing the Annual Return is usually 31st December of the succeeding year unless the CBIC extends the due date. As per Rule 101, the audit is to be conducted for a financial year or multiple thereof.
So, there is no concept of time span to issue notice -Form GST ADT-01 under sec 65 after 2 years of annual return is filed.
Regarding the second query, as per Explanation u/s 65, the expression "commencement of audit" shall mean the date on which the records and other documents, called for by the tax authorities, are made available by the registered person or the actual institution of audit at the place of business, whichever is later.
The actual institution means, the ADT-01 is issued intimating to visit the business premises, and verification of books of accounts starts when all the required books of accounts and documents are produced on the date of visit.
So, the effective date of the audit commenced after complying with all documents and clarifications for audit queries needed.
The audit is to be completed within 3 months from the effective date.
Invoking of Sec. 73 or 74 has no relevance to the time span for the issue of ADT-01.
Challenging the Notice -ADT-01 can be made if you have any other reasons.
The above are my views only.