I would say you have to definitely be lead of the lord when going on a fast for a long period of time. We see fasts in the bible (Moses, Elijah, Jesus) where fasts lasted as long as 40 days with no food or water. But it is safe to say in these instances that God himself instituted those fasts and he gave them the physical stamina needed to endure. I don't think you can fast or pray to much, and I agree with what bboy said. Out of the responses you gave, I believe the one about opening yourself up to the devil, because many people start out with the right intentions, but the devil only needs a small opening to come in. God can begin to work in someone's life or start revealing things to them, and pride creeps in and they begin to lose focus and glorify themselves instead of glorifying God. It's a thin line sometimes, between being highly anointed and used of God, and being lifted up in self. We also see stories all through the bible of people getting lifted up in themselves and trying to eclipse God's glory (God allowed Nebuchadnezzar to literally lose his mind -- he wasn't fasting or anything, but that pride thing is shown clearly in his story). I don't know if you'll find the answer your seeking, but I believe that when fasting and seeking God, you gotta stay in the Word to remain on course, and stay prayerful and make sure everything lines up with what God has already said in his Word. If we aren't sure it is God speaking, there's nothing wrong with continuing to seek Him and telling him that we are unsure, so he can help give us clarity. Anyway, this is a little long for my second post ever, LOL!! So I'll stop here.