All minor version updates are free (2.0,2.1, 2.2, etc...), and major version updates (3.0 for example) would be offered at a discount.
That being said, the version 1.x platform lasted 8 years, and the version 2.x platform might actually last longer as I have no plans to do a rewrite to 3.x at any point in the next several years. So at the very least you are good for another 3-5 years (possibly longer) if you purchase the desktop version now.
Note that the mobile versions (for iPad, iPhone, Android) are a separate purchase via the app stores. Updates to the apps are free once you have purchased.
Brad