About 1200 calories has been my min without having constant headaches and being irritable. I'm horrendously hungry the next day, but most of the time I can still keep it to about 2500-3500 calories the day after if I have enough protein. Last weekend though I lost it and probably had 5000 calories Friday and 4000 Saturday.
Also, my workouts are abysmal on fast days and I simply can't do them the day of or after heavy weightlifting. Not that I tried a heavy day on a fast day, but trying it the day after was futile.
I'm still 254 lbs.