The skitto zone is your zone, mate. NO data charges will be applicable no matter how long you browse the skitto app from your skitto connection. However, certain actions (like logging in with Facebook or reloading using debit/credit cards) takes you to third party sites and you might incur negligible amount of data there.
Also, do remember that you have to keep your internet on when using the skitto app, but you won’t be charged for it :D.