This is to do with the NEC3 contract.
Your answer is in clause 32.2.
It is the interval stated in contract data as the minimum submission periods (typically every month or every four weekly) but the Contractor can submit one when ever they want in between times and the Project Manager can submit a revised programme when ever they want which the Contractor would have to submit within the period of reply (stated in contract data 1).
Normally, at the interval stated in Contract Data part 1 which is typically 4 weekly or monthly.
But, the PM can instruct a submission and the Contractor can choose to submit a revised one - see clause 32.2