The first 100 hours after the release will be free for all and that means free forever, if you get the game before the first 100 hours end you will NOT have to pay to keep it. The 4.99$ price after the first 100 hours goes to preserving the game such as server costs etc. Best I can figure free thru 6/8.
You can see your licenses by clicking on your username then account details in the top right corner of Steam. Then click View licences and product key activations.