Tenure of office of President
Section 135(3) · 1999 Constitution (2004 edition)
What Constitution says
(3) If the Federation is at war in which the territory of Nigeria is physically involved and the President considers that it is not practicable to hold elections, the National Assembly may by resolution extend the period of four years mentioned in subsection (2) of this section from time to time; but no such extension shall exceed a period of six months at any one time.
This is the 1999 Constitution as published in 2004. Later amendments aren't included yet, so a section may have changed since.
Original source
Official Gazette · CFRN 1999 official PDF · printed page 69. Original document
Release: draft-c8d50d87fae9. Source scope, review and corrections