How much is enough is depend on the results of testing a program. If lots of faults are found with a set of planned tests it is likely that more tests will be required to assure that the required level of software quality is achieved. On the other hand, if very few faults are found with the planned set of tests, then no more tests will be required.
You may stop testing tested enough when you are confident that the system works correctly.
"How much testing is enough?" is "It depends!" It depends on risk, the risk of missing faults, which may become a reason of failure and cost us higher.
