Hello Rax,
It depends upon the need and business complexity ,mostly 3-Tier or N-Tier Architecture is used in Real time Applications.
Basic Purpose of Layering:
1.To make application more understandable.
2.Easy to maintain, easy to modify application and easy to deploy.
To understand how to split the layer ,pls refer
Understanding Multilayered Architecture in .NET[
^]
All the Best:)