Best bet is to go to your local detail shop, they can answer all your questions. But, here's my experience with clay...
My buddy used to detail cars and his clay would last for months and months, maybe even years!!!!!! You just keep kneading the clay, stretch it and fold it and just use a hose with a little water every so often to wash the clay and to wet the paint and rinse the vehicle, no special spray or soap is needed. I've used it for all my vehicles and have used the same clay bar for about six years now. By the way, make sure to wash the car very good before claying, obviously you don't want dirt on the surface. Oh and another thing, the clay is probably the most effective for fallout removal. It also removes light surface scratches and you'd be amazed how easily overspray is removed!!!! You can use it on your windows and rubber and plastic trim too, but again, make sure it is nice and wet!!!!