It's optional. After the initial 12 months has expired you can either carry on running what you already have or renew the license for $40 to get any new versions. So, if there's no new versions that interest you at that time, you just don't renew. Once you do renew you get 12 more months from the renewal date.
If you have an active license when XenForo 2.0 is released, you can upgrade to that at no extra cost.
I run three forums, on two I have the licence up to date, the other one I haven't because it is a quiet forum that's just trundling along.
What I do really appreciate with xenforo is that even when you don't keep the licence, they still release security patches as necessary for older versions so you aren't "ransomed" into updating for the sake of staying safe.