AO must issue notice u/s 148 within prescribed time limit. A formal notice must be served upon the assessee.
1) Normally time limit for issue of Notice period is 4 or 6 years.
4 years if the escaped Income less than Rs. 1,00,000/-
6 years if the escaped income is Rs. 1,00,000/-
2) This time limit shall be 16 years if it is related to Asset located outside India
3) But, if there is any specific direction contained in an order passed by the authority in any proceeding under act by way of appeal/ revision or by a court, in that situation there shall not be any time limit and the time limit shall be indefinite period. But, if at the time when the order which was subject matter of appeal or revision was passed, the time-limit for issuance of Notice u/s 148 had already expired, the time limit of indefinite period will not apply.