How often I Can Update my Application on App Store?

Not really.

It usually takes Apple 7 days to review an app and if you upload a new update before the old one has been reviewed you go to the back of the queue. If you need to update your app more frequently than every ~7 days you should look into setting up a web-based update mechanism. Obviously, you won't be able to update any code this way, but you can update content as much as you like.