What is the least time a person should pray each fard salaah ?

well no can give a specific time, it varies from person to another but the best salah is the one done sincerely for Allah (no Riya) , with complete concentration and taking care of Salah ahkam .So May Allah make our salah so beautiful that our life inside & out side salah is same inshallah i.e obedient to Allah.