atpl Met is valid for 5 yrs from the date of clearing written exam. atpl nav and radio are also valid for 5 yrs from the date you clear your written exam.
that means if you clear your atpl nav or radio written in jan 2016 and you are clearing your orals in any month of 2016 or any month of 2017 etc the paper will be valid from jan 2021 only i.e 5 yrs from you clearing written