Forever but if you want to keep receiving updates for new features or bug fixes you will have to pay again for those future updates. That makes it more or less a hidden subscription because Windows 10 is updated twice a year now and like with all software applications, you will have bugs and issues from time with those major updates. You will want those updates or will need them in order to keep them working properly.
But if your question is if the software stops working after a year. I think the answer is no. You just don't get new updates, seems fair.