The 1999 Constitution

Section 63

The Senate and the House of Representatives shall each sit for a period of not less than one hundred and eighty-one days in a year. 

(The National Legislature which is made up of the Senate and the House of Representatives,  shall convene and perform their legislative duties on one hundred and eighty (180) days of the calendar year at least.)

Section 64 (1)

The Senate and the House of Representatives shall each stand dissolved at the expiration of a period of four years commencing from the date of the first sitting of the House.

(At the end of a four year period, running from the day of the first ever sitting, both houses of the National Assembly shall be taken as dissolved, with no powers and no duties.)

Section 64 (2)

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 (1) of this section from time to time but not beyond a period of six months at any one time.

(In the case of physical military warfare, and the President decides that holding elections might not be a good idea, the National Assembly can continue to perform its duties after the expiration of the prescribed four year period, but can only extend the period of continuance for six months at a time.)

Section 64 (3)

Subject to the provisions of this Constitution, the person elected as the President shall have power to issue a proclamation for the holding of the first session of the National Assembly immediately after his being sworn in, or for its dissolution as provided in this section.

(The person who has been elected as President reserves the power to declare the National Assembly dissolved or commenced, by way of a proclamation.)

Section 60, 61 and 62 of the 1999 Constitution: Regulation of Procedures and Committees