FIDIC: what is the maximum duration for the contractor to reply the consultant letter as per the fidic? if it's mentioned, which clause?

FIDIC does not specify a general period for reply to letters (some actions have specific time periods, such as in relation to notification of claims but that is a separate issue).

Clause 1.3 of the Red Book refers to certain communications not being unreasonable withheld or delayed. A reasonable period, in all the circumstances of the particular communication in question, is therefore likely to be applied and needs to be considered on a case by case basis.