As in the discussion, we talked about focusing efforts on quality and how far is too far.
For me, that is a pretty vague question because it is open for interpretation. Overall it is generally based on the acceptable risk of the organization. Even that can get a little fuzzy.
It is all about the culture the organization has in how it treats its products and clients. There was a great analogy used in Explore It!: Reduce Risk and Increase Confidence with Exploratory Testing by Elisabeth Hendrickson. Paraphrasing, testing is like a fishnet, depending on how little the mesh is will determine how much you catch.
To focus too hard on an application or component to the point where the mesh is microscopic will take a lot of effort to get through one cycle. While on the other hand you can have a big mesh and only get the big ones. Now depending on the application being testing, you may have no choice to use the tight mesh. There could be potential lives at risk if something were to sneak through.
Culture comes in as well as excellent leadership in providing ways to move that net faster while still getting the same results. Through the use of purchasing tools to use, providing a collaborative environment where team members across all disciplines can work together to solve problems and in general, be supportive.
“There is always value to be discovered. Let me help you find it.” - Jeremy Berriault