That can vary with the size and complexity of the organization, since engaging your stakeholders and gathering requirements are primary activities. However, a good rule of thumb is 60 – 120 days from project start until the RFP is issued. Once responses are received, you can plan on a similar timeframe to evaluate proposals, conduct demonstrations and other due diligence, and make a final selection.
There are two dimensions to proper requirements coverage; breadth and depth. It is important to engage with users that represent all facets of the organization that will engage with, or be supported by, the system. It is equally important to ensure the conversation captures the needs of everyone, from “the shop floor” to “the executive suite.” Successful requirements coverage is measured by quality, not volume. In fact, one of the common mistakes that organizations make is to have so many requirements that they lose the focus on which requirements are important to success. Assurance in the requirements process comes from three things: investing the appropriate time and resources, engaging the breadth and depth of the organization, and having a track record of successful projects and lessons learned to use as an appropriate benchmark