How much longer does a war last than originally planned?
It is often the case that wars do last longer than originally planned for a multitude of reasons.
However, if it were possible to provide a specific and fixed answer to your question, then the length of a war would never be underestimated in the first place.
Because all of the stages are working simultaneously, any changes to user requirements halts the work on any component dependent on the component being changed. This can lead to a much longer development cycle than originally planned for. Additionally, this model requires excellent and current communication between all teams, which is sometimes not easily feasible.