I have an iPad2 my son plays on it a little too much so what I'm looking for is an app which turns off the iPad after a preset time?

Unless he's bigger than you, it's called "Takeit away from him".

There are apps available, one quick search on the store brought up a few. However I like to use what I like to call Parental Guidance, if they go over a certain time they are banned from using it for a while. Much more effective than software.

Mark