At what point should architecture become layered?

Question

Obviously, "Hello World" doesn't require a separated, modular front-end and back-end. But any sort of Enterprise-grade project does.

Assuming some sort of spectrum between these points, at which stage should an application be (conceptually, or at a design level) multi-layered? When a database, or some external resource is introduced? When you find that the you're anticipating spaghetti code in your methods/functions?

Answer 1

when a database, or some external resource is introduced.

but also:

always (except for the most trivial of apps) separate AT LEAST presentation tier and application tier

see:

http://en.wikipedia.org/wiki/Multitier_architecture