We are trying to put some statistics together internally to get a better handle on how long it takes to complete pre-project work for business improvement projects. This work includes work corresponds roughly to the "Starting-Up A Project Phase" from PRINCE2 and the "Starting the Project" part of the project lifecycle described in PMBOK. During this phase we are defining scope, high-level description of the project product(s) and high-level identification of resources so that we can develop documentation comparable to a project charter or project brief. We want to put stats together so that we can try to better time-box this phase.
Given that all projects are different, are there any official statistics or anecdotes for how much time should be spent on pre-project work compared to actual project work? We would use this information to help us gauge whether or not our processes are more/less time consuming than they should be at a high level (e.g. if there is enough opinion out there that for a hypothetical 10 month project you should spend 2 months prior to this doing pre-project work, we would revisit our processes if our pre-project work took on average 5 months to complete).