It's one time cost. You can use your purchased version of Booking Calendar as long as you need on your website.
Also, you can request free updates your version of Booking Calendar (if such updates ready) during 6 months after initial purchase of plugin.
If you will need to extend period of support and getting updates to next 6 months, it will cost 50% from the original cost of your Booking Calendar version (original cost - its cost of Booking Calendar version without other discounts).
Optional subscription of getting updates with discounted cost.
Please note, during initial purchase of plugin, its possible to select (or deselect) optional subscription (billing each 6 months) for getting Updates and Support for plugin with additional discount. First 6 month during initial purchase of plugin, getting of updates is for free. Possible to cancel subscription of getting updates at any time.
If you was not selected subscription of getting "Updates and Support" during initial purchase, and if you will need (in a future) to extend period of support and getting updates to next 6 months, it will cost 50% from the original cost (original cost - its cost of Booking Calendar version without other discounts).
You can request new update of Booking Calendar on this page.