Can I take niqab on and off, i.e. according to situation and where I go?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Wearing Hijab that includes covering whole body and face is an obligatory part of Shariah. A believing woman is not free to take it off or abandon it. Allah legislated Hijab to safeguard the dignity of a woman and prevent her from being harmed and also to ward off the dangers and temptations that certainly occur from both men and women. Allah says: {O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies (i.e.screen themselves completely except the eyes or one eye to see the way). That will be better, that they should be known (as free respectable women) so as not to be annoyed. And Allâh is Ever Oft Forgiving, Most Merciful.} [33:59]
Therefore, unless a Muslim woman strongly believes that wearing Hijab will cause her physical harm she is not allowed to take off her Hijab in front of alien men regardless of the place or time since; wearing Hijab is a form of worship, a subjection to the commands of Allah besides being a sign of chastity. It also protects a woman, and shelters her from the lustful gaze of men.
Allah says: {….. And when you ask (his wives) for anything you want, ask them from behind a screen, that is purer for your hearts and for their hearts…… } [33:53]
Indeed, Allah ordered the wives of the Prophet (Sallallahu Alaihi wa Sallam), who were, the best of all women to wear Hijab in front of the companions of the Prophet, the best people other than the Messengers of Allah. The requirement of Hijab is more relevant now, since, the temptations, adultery, dissoluteness and all kinds of evil are so wide. Allah knows best.