Level 3
It takes a little time to write the tests, but using them over and over will help you in the long run. Because you won't have to manually input data etc into fields, when performing tests.
You should write a test at the very least, every time you accept user input and has to validate or use data etc. The test should make sure that its not possible to break, hack or any way input data that you dont want, and if you had to do this every code change, the time spent writing the test, is saved back from using it.