Hi all. I am a commercial photographer but have started to shoot a few promotional/instructional video's with a couple of GH1/GH2's. Now an existing client has asked me to reproduce something like this.

Timelapse - How to build a mezzanine floor by Spaceway - YouTube

What would the best way to achieve this? Some parts of my clients installations would take about 20hrs and the total time of the project would be about 4 weeks. I obviously cant be on site for that amount of time so I am wondering if anyone has got any advice.

If you can be 100% sure you can get the same camera placement you can keep going back and setup the camera in the same spot and shoot for a few hours. Or alternatively shoot for as many hours as you can and then the next time get a nice framing and shoot for more hours. Then it looks like you meant to do the cuts between multiple time lapse cameras.
Just some initial thoughts.

You need a camera that can plug into an outlet for constant power and a card (or HHD) large enough to handle all the data.

Moving the camera no matter how precise you are is likely to result in a shift in the field of view.