Irrespective of your approach or methodology and the type of project to be handled, Drupal requires preparation. From objectives or goals to the time frames, you need to be quite clear with what the client requires and this is possible only if you and your client are on the same page. Inputs from your team who are experts in Drupal will be your trump card in the process of planning and preparation. This will not only enable you to come up with better time frames, you would also be able to know the prerequisites, inter dependencies and the work condition.
Never fail to miss critical inputs irrespective of how small they are to the Drupal project. You might end up missing something valuable.