How much unit testing is a good thing :
Unit Testing is not static that once you have done and your job is complete, It will go on through out life of product until you do not stop further development on your product
Basically Unit Testing should be done each time :
1) you do a fix
2) New Release
3) Or you find a new Issue
I have not mentioned development period, as during this period your unit level test are evolved.
Basic thing here is not Quantity (How much) but coverage of your unit test
For example : For your application you fond a issue an particular function X, You do a
fix for X, If no other module is touched you can do unit testing
applicable for module X , Now this is point how much unit testing for X
cover
So Your unit Test must check:
1) Each interface
2) All input/out operations
3) Logical checks
4) Application specific results
