The Lisbon Treaty simply refers to member states sovereign ability to trigger article 50 in the first place - what follows afterwards when they are no longer members of the EU is a different matter. At any rate, the UK has always been free to accept or reject the backstop -it is a sovereign choice- just that it must bear the political and economic consequences if it chooses to reject it. It makes no sense to time limit the backstop - it violates its insurance function never mind that as soon as you put an arbitrary number on it, it’ll be red meat for the swivels. What matters are the robustness of the processes regulating the UK’s ability leave the backstop, once in it, ensuring that neither party is acting in bad faith.