It is crucial for large-scale projects to have a consistent interface despite the amount of people who independently work on it.
A corporate standard is a must in such cases. It includes the rules for creating an interface as well as a set of commonly used elements (i.e. brand images, navigation elements, common blocks of elements).
Standard for UI (and sometimes UI brand book).
4-12 weeks depending on a scale of an interface and the number of elements used.