A Sadhaka gains full mastery over a particular Mantra after obtaining it from a Guru and doing the japa as prescribed.
This process is often called "Mantra Sadhana" or "Mantra Puruscharana".Now the Mantra can be of any Deity, depending on the nature of the Sadhana or the choice of the Sadhaka, like Kali,Tara,Shiva,Vishnu etc.
Description of the process of Purascharana is an important part of the Agama(Tantra) Texts.
After the Sadhaka obtains such Mantra siddhis he can make the Mantra to work at his command to create even miracles like curing diseases of others etc.
My Question-Is Mantra Siddhi thus obtained temporary(say it stays with the Sadhaka for only "X" number of years but not after that many years have passed) or it stays with the Sadhaka till his death?