The proper officer shall, within a period of six months from the date of communication of the provisional assessment order, pass the final assessment order after taking into account such information as may be required for finalizing the assessment. However, the time limit of six months can be further extended on sufficient cause being shown and for reasons to be recorded in writing in the following manner:
(a) by the Joint / Additional Commissioner for a further period of six months;
(b) by the Commissioner for such further period not exceeding four years.
(ICAI FAQ PUBLICATIONS 06-09-2017 PROVISIONAL ASSESSMENT: FAQ NO. 4)