For libraries, tools, and frameworks, testing is crucial as it ensures that the code relying on them can address the issue at hand. Code can only be as reliable as what it's leaning on. So, to answer your question, a lot of time.
In a business-oriented project(at most jobs), code may undergo frequent changes due to requests from business, thus too much focus on testing could potentially slowing down development speed if extensive testing is implemented for each change.
However, regression tests can still provide valuable insights and allow for faster development later in the life of the project.
While many projects only focus on happy path testing, the use of such tests might not be as high. Coupling them with Negative Testing, and even better, implementing boundary testing, compels developers to consider both valid and invalid inputs, helping to identify and address potential edge cases before they become bugs or security issues in production.
For instance, this [0] codebase has more tests than actual code, including fuzzing tests.
In a business-oriented project(at most jobs), code may undergo frequent changes due to requests from business, thus too much focus on testing could potentially slowing down development speed if extensive testing is implemented for each change. However, regression tests can still provide valuable insights and allow for faster development later in the life of the project.
While many projects only focus on happy path testing, the use of such tests might not be as high. Coupling them with Negative Testing, and even better, implementing boundary testing, compels developers to consider both valid and invalid inputs, helping to identify and address potential edge cases before they become bugs or security issues in production.
For instance, this [0] codebase has more tests than actual code, including fuzzing tests.
[0]https://github.com/Attumm/redis-dict/blob/main/tests.py