The number of times a Mantra is required to be chanted during Purascharana depends on the number of letters the Mantra is formed of. It is usually N lakh where N denotes the number of letters the Mantra has.
For example, the Gayatri Mantra is 24-syllabled. So, 24 lakh is the prescribed number for it's Purascharana.
So, for mono-syllabled Mantras (like aim, klim etc.) it is one lakh.
The Gâyatrî is to be repeated also. The food offered by a thief,
Chândâla, Ksattriya or Vais’ya is very inferior. The food of a S’ûdra,
or the companion with a S’ûdra or taking food in the same line with a
S’ûdra leads one to suffer in the terrible hells as long as there are
the Sun and Moon. The Puras’charanam of Gâyatrî is repeating this
twenty four lakh times (i.e., as many lakh times as there are
syllables in the Gâyatrî)
Devi Bhagavatam 11.21
In general, irrespective of how many letters the Mantra is made of, at least one lakh Japa is required for attaining Mantra Siddhi.
This is mentioned by Swami Sivananda:
Get the Mantra initiation from your Guru. Or pray to your Ishta Devata
and start doing Japa of the particular Mantra, if you find it
difficult to get a Guru.
- METHOD OF MANTRA PURASCHARANA
The repetition of a Mantra with rigid spiritual observances a fixed
number of times to obtain quick spiritual progress is known as Mantra
Purascharana. It can be performed for material progress too. The
practitioner should observe certain rules and undergo strict dietetic
discipline to ensure quick Mantra Siddhi.
During the Purascharana take only fresh vegetables, fruits, milk,
roots, barley and Havis-Anna (rice cooked with ghee, sugar, milk). A
Sadhaka can live on pure Bhiksha (alms) also. If you can live on milk
alone during the period of Purascharana it is highly laudable. You can
have Mantra Siddhi even by repeating the Mantra a lakh of times.
Select any holy place of pilgrimage on the banks of sacred Ganga,
confluence of rivers, mountain valleys of charming scenery, temples,
Tulasi gardens, below Asvattha trees or convert a portion of your
house into a temple by keeping the picture of the Lord, burning
incense etc., and by suitable decorations. Purascharana done in holy
places has a benefit hundred times superior to that done in one's own
You can select any Mantra for Purascharana. Your Guru Mantra or Ishta Mantra is the best. Sandhya time, sunrise, sunset, midday are
all recommended for Japa. Repeat the Mantra as many lakhs of times as
there are letters in the Mantra. You can do half of that number. In no
case the number should be less than a lakh.
Sit facing East or North during Japa. Select Siddha, Padma, Svastika
or Virasana for Japa. Never sit for Japa with a loaded stomach. Have
fixed timings for Japa. Take a bath before you start, if possible, or
at least wash hands and feet. Perform Achamana or sipping of
Sanctified Mantra water. Deer skin, cloth, blanket, Kusa grass or
tiger skin can be used as seats while doing Japa. Spatika, Tulasi,
Rudraksha Malas can be used for counting the number of Japa. Have a
Mala with 108 beads or half or one-fourth that number.