You can charge anytime you want providing you see your ennemy and there is a practicable 10-feet long straight path between you two.
D20SRD says :
You must move before your attack, not after. You must move at least 10 feet (2 squares) and may move up to double your speed directly toward the designated opponent.
You must have a clear path toward the opponent, and nothing can hinder your movement (such as difficult terrain or obstacles). Here’s what it means to have a clear path. First, you must move to the closest space from which you can attack the opponent. (If this space is occupied or otherwise blocked, you can’t charge.) Second, if any line from your starting space to the ending space passes through a square that blocks movement, slows movement, or contains a creature (even an ally), you can’t charge. (Helpless creatures don’t stop a charge.)
If you don’t have line of sight to the opponent at the start of your turn, you can’t charge that opponent.
About the charge, it also says on the Fleet of Foot feat :
Without this feat, you can run or charge only in a straight line.