Where does one find the time stated for the period for reply in communications in NEC3 contracts? Is it 1 week, 2 weeks or some other period?
The period for reply is stated in italics and it is therefore an identified term. Identified terms are shown in the contract data (for example see clause 11.1 of the NEC3 Engineering and Construction Contract (ECC)), which has to be completed at the tender stage by the employer (part one) and the contractor (part two). In this case it has to be filled out by the employer in contract data part one, so the answer to your question will depend upon the period selected by the employer.
Replies required by the ECC have to be made within the period for reply, unless another period is stated in the contract (see clause 13.3).