Extent means depth at which particular controls should be checked, how much time , human resources and items should be checked.
in some controls we may do loads of checking in some we may do less checking it depends on following factors
FLERT

The Frequency of the performance of the control by the entity during the period.(Stock count is weekly Vs Fixed asset count is half yearly)
The Length of time during the audit period that the auditor is relying on the operating effectiveness of the control. (More time more checking)
The Expected rate of deviation from a control. (If expected rate is very close to tolerable rate of deviation then we have to check more of that control, to extra sure)
The Relevance and reliability of the audit evidence to be obtained regarding the operating effectiveness of the control at the assertion level. (Double payment is big risk so software feature to detect same number of PO is relevant , so check more)
The extent to which audit evidence is obtained from Tests of other controls related to the assertion. (If other controls on that assertion are not effective do more checking here)