As Viach Kakovskyi (@BackendandBBQ) stated in his blog post "If you do not have time to write tests today - you will find the time for fixing bugs Friday’s night". In other words, to establish solid reliability in production tomorrow we need to invest our time today.
Your need for tests for your current project depends on:
- The size of the team that maintain the codebase
- Size of the codebase
- Duration of development and maintenance phases of the project.