ERP is not about deploying once and forgetting. ERP software is implemented so that organization can get benefited by increased efficiency. However the efficiency depends on how the Business Rules are managed, how the users follow ERP processes & how the data is being utilised by organization to continuously enhance the efficiency.
One specific apprehension with ERP systems is that over a period of time changes in the ERP Administration team and induction of new employees who are not well trained in the ERP processes leads to dilution of discipline and controls built into business processes, leaving the company vulnerable.
The ERP Audit should be done on annual or biannual basis.