A cup with a hole in it is good enough, as long as the chameleon has the water for at least an hour or two.
I mist for 1-2minutes because my guys are good drinkers. Some chameleons need at least 5 minutes (and different species require different times, but I'm assuming you have a veiled or panther.) just keep spraying until he stops drinking then spray the plants away from him for a little longer (unless the humidity doesn't raise enough in that time, then for a while longer, or until you think it's good enough.)