Not an easy question to answer, as there are many variables that can affect the amount of work that goes into managing the certification process. What country or region are you working in? I'll offer an estimate for a project in the USA as a starting point and hope others chime in.

For an experienced LEED AP, working with a design and construction team that is familiar with LEED requirements and procedures, anywhere from 300 to 500 hours would not be uncommon for a mid-sized commercial project. We've also been hearing that for projects less than 60,000 square feet the amount of time and effort for documentation doesn't go down significantly for smaller projects. It would be great to hear other people's estimates of the time they've spent.

For projects outside the US & Canada, there may be less "existing infrastructure" of people and companies familiar with LEED requirements and procedures. Factors that can add to the time and effort:

- Any contractors and product suppliers who are unfamiliar with LEED will need some coaching on what new information they need to provide for MR and IAQ credits;

- Extra time for the LEED AP to finding out if materials and systems meet LEED requirements if there are few LEED projects previously done in your area;

- Commissioning: For CxA new to LEED, extra time to develop the Cx plan and procedures;

- Energy Modeling: LEED documentation format and requirements may differ from standard practices in your region;

- Other factors that can also add to your time and effort: multi-use buildings or a non-typical program, higher level of LEED being pursued, a fast schedule needing close coordination, team members who are un-enthusiastic about LEED...

Many firms have experienced a fair amount of extra time, effort, and frustration on their first LEED project, but after going through that the first time the experience can benefit your future projects and help establish your credentials.

Factors that can help your effort:
- Forming a local or regional user group of LEED AP's or people working on LEED and green projects to share information, lessons learned;

- Contacting World Green Building Council in your region for contacts, information, or resources in your area;

- Working with a foreign or local LEED consultant familiar with your area can save a lot of time and effort on research and backtracking; they also help guide you through the steps;

- Using the LEEDUser forum and resources!

By the way, you mention pursuing Core and Shell for a residential building - not sure if that's been done for a residential project, which is usually has most of the interior walls and finishes; CS is usually used for speculative office buildings that have few of the interior office walls or finishes other than core bathrooms and service areas. Check the BD&C Rating System or Reference Guide introductions, and MPR Supplemental Guidance document.