It depends on your project.
Here is the guidelines I use.<p>Prototype / Early Stage Project.
Avoid TDD during early stages. Focus on learning and make the project work.
Also get feedback from potential users.
At this point, only one or two developers should be involved.<p>Mid Stage / Concept proven but still a lot of work needed.
Concept is proven. Heavy development is going on.
More developers 2+ are working on the project at the same time.
Have unit/integration tests for most important features.
Code coverage is less than 20%.<p>Beta / Release stage
Concept is proven and your application is ready for consumption.
Have solid unit tests and integration tests.
May have a lot of developers 5+ and geographically distributed.
At this stage, each feature development should go with TDD.
Code coverage should be 80%.