I do the testcases based on where the project is at that point in time.
Here are the three stages, that can help you decide how much tests needs to be there.
[1] Initial stage where we are trying to make things work. At this stage code base is very small < 1000 lines. This is like prototyping. It works with limited functionality. No tests needed at this time.
[2] Heavy development phase. At this stage, we have proved the concept. Now we are adding a lot of new features. We identified some features as must have. Also, code is getting re-factored/re-designed based on what we learn. At this stage, we add tests for the must have functionality. Thus, we can ensure that important features are not broken by newer code.
[3] Mature phase. The code is mature. Most of the features are working fine. Code base may be large 100000+ lines. At this stage re-factoring/re-designing is not easy. Mostly incremental changes are happening. At this point, we should have upwards of 70% code coverage. Typically, the test code will be more than the code when we have 70%+ code coverage. But, it is very important to have tests, since it ensures that all features are tested even when a minor code change is made.
[1] Initial stage where we are trying to make things work. At this stage code base is very small < 1000 lines. This is like prototyping. It works with limited functionality. No tests needed at this time.
[2] Heavy development phase. At this stage, we have proved the concept. Now we are adding a lot of new features. We identified some features as must have. Also, code is getting re-factored/re-designed based on what we learn. At this stage, we add tests for the must have functionality. Thus, we can ensure that important features are not broken by newer code.
[3] Mature phase. The code is mature. Most of the features are working fine. Code base may be large 100000+ lines. At this stage re-factoring/re-designing is not easy. Mostly incremental changes are happening. At this point, we should have upwards of 70% code coverage. Typically, the test code will be more than the code when we have 70%+ code coverage. But, it is very important to have tests, since it ensures that all features are tested even when a minor code change is made.