Japa is the repetition of the name of that in which we believe.
Japa is essential.
Japa is not simple words - each divine name is full of divine vibrations. These surround us and protect us and penetrate both our bodies and our whole inner being. Remembrance of the divine name gives immediate peace and happiness and turns us from the worldly to the Divine.
There is no special and limited time for japa. It is very good to do japa all day. If this is not possible then remember whenever it is possible.
We can practice japa during all activities. It is easier to remember when we do physical work without mental work. This japa helps us to purify our consciousness and make our sadhana easy.