Is there any time limit for filing appeal

Yes. Time limit to file appeal before Appellate Authority, for an assessee (person aggrieved) is 3 months from the date of communication of the order. Whereas in case of an appeal filed by the department on the basis of the directions by the Commissioner shall have to be filed within 6 months from date of communication of the decision or order.

(ICAI FAQ PUBLICATIONS 06-09-2017 APPEALS AND REVISIONS : FAQ NO. 5)

Under what circumstances Commissioner could direct the Officer to prefer an appeal against the order of the adjudicating authority

The Commissioner may, on of his own motion, or upon request from the Commissioner of State tax Commissioner of Union Territory tax, call for and examine the records of any proceeding in which an adjudicating authority has passed any decision or order under this Act or the SGST Act or the UTGST Act, for the purpose of satisfying himself as to the legality or propriety of the said decision or order. The Commissioner may by an order, direct any Officer subordinate to him to apply to the Appellate Authority within six months from the date of communication of the said decision or order for the determination of such points arising out of the said decision or order as may be specified by the Commissioner in his order.

(ICAI FAQ PUBLICATIONS 06-09-2017 APPEALS AND REVISIONS : FAQ NO. 4)

Who could prefer an appeal before the Appellate Authority

Following could prefer an appeal before the Appellate Authority:
(a) Any person aggrieved by the order or decision of the adjudicating authority
[FORM GST APL-01Chapter-XIII – Appeals and Revision of the CGST Rules]
(b) Any authorised officer on direction of the Commissioner (this is referred to as application and filed in FORM GST APL-03).

(ICAI FAQ PUBLICATIONS 06-09-2017 APPEALS AND REVISIONS : FAQ NO. 3)

Who is an adjudicating authority

Section 2(4) defines adjudicating authority to mean any authority appointed or
authorized to pass any order or decision under this Act. However, following are not covered under the ambit of adjudicating authority:
(a) Central Board of Customs and Excise (Board),
(b) Revisional Authority,
(c) Authority for Advance Ruling,
(d) Appellate Authority for Advance Ruling,
(e) the Appellate Authority
(f) Appellate Tribunal
Therefore, an order passed by any officer other than the revisionary authority as well as other authorities mentioned above could be contested before the Appellate Authority.

(ICAI FAQ PUBLICATIONS 06-09-2017 APPEALS AND REVISIONS : FAQ NO. 2)

Can the Advance Ruling pronounced under Section 98 or 101 be rectified

Yes. The Authority or the Appellate Authority may amend any order passed by it under section 98 or section 101, so as to rectify any error apparent on the face of the record, if such error is noticed by
 the Authority or
 the Appellate Authority on its own accord, or
 is brought to its notice by the concerned officer,
 the jurisdictional officer,
 the applicant or
 the appellant
within a period of six months from the date of the order:
However, no rectification which has the effect of enhancing the tax liability or reducing the amount of admissible input tax credit shall be made unless the applicant or the appellant has been given an opportunity of being heard.

(ICAI FAQ PUBLCATIONS Advance ruling under GST: FAQ NO. 20)

What are the powers of Advance Ruling Authority and the Appellate Authority for Advance Ruling

The Advance Ruling Authority and the Appellate Authority for Advance Ruling shall have all the powers of the Civil Court regarding;
(a) discovery and inspection;
(b) enforcing the attendance of any person and examining him on oath;
(c) issuing commissions and compelling production of books of accounts and other records. Further, the Authority or the Appellate Authority shall be deemed to be a civil court for the purposes of section 195, but not for the purposes of Chapter XXVI of the Code of Criminal Procedure, 1973, and every proceeding before the Authority or the Appellate Authority shall be deemed to be a judicial proceedings within the meaning of sections 193 and 228, and for the purpose of section 196 of the Indian Penal Code. Further, if the Advance Ruling Authority and the Appellate Authority for Advance Ruling finds that advance ruling has been obtained by the applicant or appellant by fraud or suppression of material facts or misrepresentation of facts, it can declare such ruling or Order to be Void ab-initio.

(ICAI FAQ PUBLICATIONS Advance ruling under GST: FAQ NO. 19)