Within Vajrayana Buddhism, considered to be an accelerated path to Enlightenment, the Ngöndro preliminary practices are viewed as essential. Below is a brief but detailed Buddhism video documentary on the essentials of the Ngöndro spiritual practice:
As we can see, mantra recitation is considered to be an absolutely fundamental component of these spiritual practices. Practitioners are required to recite the mantras hundreds of thousands of times, or even a million times or more. This is estimated to take around 1500 hour. Hence, to complete such practices can take six months if one enters a dedicated retreat, or else several years if mixed in with normal daily life.
There are other key requirements, which the above buddhism discusses.