Depends completely upon the developer and their own preference. In the Mobihand control panel we have an option that lets us set "Upgrade Rules" based upon what version a user has, when they purchased the app, and what version they're upgrading to. That way we can require users to pay for an upgrade if it's a major change or allow it to be available for free if it's just a minor one.
But like I said it depends on the app. The option to charge for updates is there, but not all of them will use it. I personally would not charge for any updates once the user has purchased the app, and there are many developers like this, but there are also apps that will include updates for a year or stuff like that. Contact the developer of the app in question and ask them about how long updates are included.
