The Deputy President's term of office is not fixed by law.
The Deputy President's term begins when he or she is appointed by the President from amongst members of the National Assembly and takes a prescribed oath.

The Deputy President's term is ended by one of four constitutional mechanisms: dismissal by the President, a successful 'motion of no confidence in the President' by the National Assembly, a successful 'motion of no confidence excluding the President' by the National Assembly, or a newly elected President's assumption of office.
Presumably, a statement of resignation would also be sufficient to end a Deputy President's term of office.
The Deputy President's term of office is not fixed by law. The Deputy President's term begins when he or she is appointed by the President from amongst members of the National Assembly and takes a prescribed oath. 
The Deputy President's term is ended by one of four constitutional mechanisms: dismissal by the President, a successful 'motion of no confidence in the President' by the National Assembly, a successful 'motion of no confidence excluding the President' by the National Assembly, or a newly elected President's assumption of office. Presumably, a statement of resignation would also be sufficient to end a Deputy President's term of office.
Deputy President of South Africa