Comment by flyinglizard

It becomes legacy once the organization understands that the codebase, or platform, or business case, is obsolete. It does not mean it's immediately phased out; it may continue working for decades more with some love and attention, but it's clear that it is in the past rather than the future. The easiest way to tell if a project is legacy is how desirable it is for programmers in the company to work on.