Well, I've done 30 day water fasts (zero calories) on a couple of occasions and one to three week ones more often.
I have, though, given it up for weight loss purposes (just a personal decision).
Of course, as a disclaimer I have to warn that you should only do this under a doctor's supervision.
Now, a week is easy--in fact, after the first couple of days on water, it gets really easy for at least two to three weeks as your hunger pretty much goes away. It becomes, as time goes by, more of a mental hunger rather than physical hunger.
I continued my normal daily routine during my fasts--you do need to be a little careful as you can get dizzy, especially when standing up quickly.
The worst part for me was the muscle cramps that start up after a while. You've got to drink tons of water (within reason--no water poisening) to keep hydrated and you'll wear a path to the rest room.
I personally could not stomach any vitamins or supplements so I'm sure I had all sorts of deficiencies going on.
Be careful when breaking the fast--take a few days to work up to solid food.
If you have any questions, post up. I am not an expert, doctor, or anything other than a guy whose done this a few times so take any advice FWIW.
47 M 5'8"
May 4, 2010...... 440? lbs. (Start FitDay Size 60 Jeans)
June 19, 2010.... 393 lbs. (First Weigh-in)
June 19, 2011.... 229 lbs. (164 lbs. gone in one year :-)
Current Weight... 185 lbs. (Size 36 Jeans)
Next Goal 169 lbs. (07/04/12)
Ultimate Goal 165 lbs. (12/31/12)
The best exercises for weight loss are Fork Putdowns and Table Pushaways.