I've done up to 7 days. This was probably a 3-5 day fast.
I didn't notice any decline until I tried to solve a programming problem, and the part of my brain that normally jumps in and figures the hard stuff out just wasn't there.
I can concur. Used to do long fasts when I was in high school, so tried doing it again in the past couple of years, and suddenly I couldn't function at work.
That being said, I've successfully gone 18-24 hours without eating multiple times without any real noticeable detrimental effects on my ability to concentrate on programming or get work done, especially if I time it so I stop eating at 6pm one day and start eating again after 12pm the next day.
