Any assessment of time is essentially any delay which is due to the compensation event. To assess the effect you need to start with a programme which is realistic, so use the Accepted Programme and update it to the point in time when the CE occurred. You then add the effect of the CE to this updated programme to show how the CE has caused delay. The period of delay is assessed as the effect on planned (Sectional) Completion Date or planned Key Dates.
The NEC produced a guidance document, which you may find useful, titled;
ECC PRACTICE NOTE 1 - October 2017
Assessing delays due to compensation events.
An assessment for a CE uses defined principles but is not an ‘exact science’. If you don’t think the Subcontractor has assessed the quotation correctly then you can make your own assessment. The ‘prospective or retrospective’ argument is always a difficult issue, although if the matter has not been assessed correctly anyway, then you would need to apply the ‘correct’ principles of assessment anyway which would mean correcting the logic within the programme as a starting point.