RUP project planning

RUP has 4 phases and the project has to go through that phases. But when i am planning the project i have to give an estimated time to complete the project. RUP says to plan for the next phase in the current phase. in this scenario how can plan for the whole project. If i use UCP method to estimate time, i have to have use cases written. At the initial phase (inception), i don't have use cases written. is there any efficient method to estimate time or i have misunderstood RUP?