Generally speaking shastras talk about doing 1,25,000 japa of any mantra to obtain mantra siddhi. And then Vedic Pauranic literatures and their guru traditions talk about other numbers derived from counting the number of beejas in your mantra, to decide the japa required.
Along with the mantra for which sadhaka wants to attain siddhi, they should also have done jaapa of Bhairava, Ishta, and Guru Mantra along with certain required beejas.
Sadhana is a journey of many lifetimes. It does not come easily, and does not come to those who want to bypass cosmic procedures.
Adhikara is most important in trying to obtain kripa of any Ishta. Not every jeeva has adhikar for all kind of siddhis. Only devtas for whom jeeva has already done some journey in previous life, provide quick kripa, otherwise if jeeva is doing upasana, sadhana for the first time, then it is possible that this entire life they may not see anything, but if a jeeva attempts an upasana, there are divine bodies which will ensure that jeeva completes that sadhana in this or next lives, if jeeva remains firm in their resolve towards their ishta.
Those who start a sadahan and then run away from it, ruin their journey, make their guru patita, and at the same time cause immense injury to repute of their Ishta as well. Therefore always prepare yourself fully before starting any sadhana, and then give your entire life towards achieving pratyaksheekaran of your Ishta.