Sri Aurobindo does not believe much in mechanical methods and stressed more on the attitude and the inner state rather than on the outer means and formal methods. Reciting a mantra as indeed everything else should be an act of love, a means to offer oneself to the Divine or to prepare our consciousness for the Divine. Knowing the meaning helps as it then includes the higher mind and not just the mechanical physical mind and its energy in the invocation.
There is no such rule of time of the day or the number of times. However, it is best to set a certain convenient time and place apart as it facilitates the forces to align in a particular way. There is no such rule that the practice of mantra should be restricted to those moments only. Why not remember God as often as we can as and when we can. Even in the ancient Indian discipline the time advised was basically because early morning hours and evening twilight moment are two such periods when nature, physical nature itself is quiet and therefore it facilitates the inward concentration. Later these things began to be enjoined as a mechanical rule and thereby it lost the deeper truth behind it.
The same applies to place. Since each place contains certain vibrations and formations that hang around it depending upon the people and the transactions of consciousness that have taken place there, it is best to set apart a space meant for meditation and dedicated to connect us with higher states of consciousness and with god. Later as we grow and our consciousness deepens and widens we can remain perfectly connected in all conditions and circumstances since the Divine is indeed everywhere and at every moment and in every activity. But in the beginning and for a long time it helps to make certain rules for oneself provided they are not rigid and one does not make a fetish of them. Rules are meant to help us and not to enslave us. And it is best for each one to make these rules for themselves and keep them plastic enough to mould according to genuine needs of the play of life.
