How Much Testing is Enough?

1. It is possible to do enough testing, but determining how much testing is necessary is diffcult to understand.
2. enough testing can be determined or confirmed by assessing the results of testing. 
3. If lots of faults and errors are found in the test than it is more likely that more number of test cases will be required to assure the quality of the product.  

Its acually never enough. The amount of efforts put on testing varies according to the commitments and budget.

Ideally, its responsibility of a seller to quantify testing effort and stop it when he/she himself or herself is satisfied with the quality of the software product while looking at it as a buyer.