How detailed a set of requirements do you require from us?

Again, it's an interactive process. In scenario one: if you do not have any spec outlined we ask you to give us minimal requirement specifications (may be a page or two page description of the system). Then we start developing the formal requirements document from that. Every release of this document is reviewed by the client and we do modifications accordingly. Generally it goes on from few weeks to a couple of months (depending on size of the project). In this process the requirement specifications becomes the bible for development. All changes and modifications are captured continuously in this document. On the other hand if you already have a specification document we start with a discussion on that, refine it for some period of time, try to freeze it once all agreed.