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 purchase of plugin. You can request new update of Booking Calendar on this page.
If you will need to extend period (of support and getting free updates) to the next 6 months, it will cost 50% from the original cost.