My role as a UX designer consists of research on the target demographics, type of business and discussing everything with the clients to define the purpose of individual steps and possible problem points. Then I craft some proposals based on the info I've gathered and then we iterate to get as close to the optimal result as we can. When the visuals are polished the developers step in and we make some adjustments if needed, still taking all the info and previous work in consideration.
The duration of the process depends greatly on the gathering of the info and discussions and this varies from project to project.