Absent from the Feast - December 2016. As far as I am aware, there is no definitive answer to that question though withdrawal of the notice by mutual agreement would seem possible.
Article 50 has received a considerable examination in a paper by Piet Eeckhout (University College London, Faculty of Laws) and Eleni Frantziou (University of Westminster) - Brexit and Article 50 TEU: A constitutionalist Reading.
The Abstract reads: "This
article considers the constitutional requirements and implications of
Article 50 TEU for the European Union. It argues that it is essential to
read Article 50 in light of the inherently constitutionalist features
of the Treaty of which it forms part together with its drafting context,
that of the Convention on the Future of Europe, as well as the
substantive protections of EU constitutional law. The article
demonstrates that substantial constitutional constraints are in place in
EU law, which can affect four of the most significant debates in the
withdrawal process, namely: the manner in which notification to withdraw
from the Union is given; the revocability of a decision to withdraw;
and the legal basis of the withdrawal agreement. These debates raise
complex matters of EU constitutional law that must be determined in
order for the Article 50 process to be conducted in accordance with the
joint UK and EU commitment to respect the rule of law."