The word mantra in Sanskrit means ‘that which protects by being meditated upon’ (mananAt trAyate iti mantrah).
Mantra can contain bijAksharAs and name of the deity.
Example: chAmundA mantra ॐ ऐं ह्रीं क्लीं चामुण्डायै विच्चे।
Ganapati mantra : ॐ Gam Ganapataye Namaha
or may contain words like gAyatri mantra:
ॐ भूर् भुवः स्वः ।
भर्गो॑ दे॒वस्य॑धीमहि ।
धियो॒ यो नः॑ प्रचो॒दया॑त् ॥
Coming to the question part, the OP asked about the effects/Siddhis/changes after reciting a mantra for certain times on daily basis.
It is a very tricky question indeed.
One must understand that the purpose of mantra,ie., daily recitation of particular words/sounds for certain times or meditation on a image, etc, is to train the mind to hang on to a single idea, but not to attain powers/siddhis or concentration, etc. One has to discard that single idea finally to attain ABSOLUTE BLISS.
Attaining powers/siddhis or concentration, etc, may occur as a consequence of doing mantra japa or meditation on a form, etc.
In the case of some persons, mantra japa or meditation on a form, etc, may result only in ABSOLUTE BLISS but not in attaining powers.
It depends on one's prArabdha (stored merit of the body).
Any mantra should be learnt from an accomplished person in a particular mantra, otherwise recitation of a mantra will be equal to a nAma japa (recitation of sacred name like Sri Raama) only, but not a mantra japa.
A small but a vital hitch is to be noted here.
An accomplished person in a particular mantra will never reveal the secret of recitation to every person, but wait for a person who has reached certain maturity levels.
If a matured disciple is available, the accomplished guru will catch hold of him and teach it to him.
To understand this concept, please read Sri Ramakrishna Paramahamsa and Swami Vivekananda life histories. Sri Ramakrishna Paramahamsa imparted his knowledge completely to Swami Vivekananda only.
Consider the case of Sri Kaavyakantha Ganapati Muni, a disciple of Sri Ramana Maharshi.
A great scholar, a performer of varieties of mantra japa, yet remained unsatisfied due to non-realisation of all his actions, till he met Sri Ramana Maharshi. Consider his own words when he met Sri Ramana Maharshi.
"All that has to be read I have read. Even Vedanta Sastra I have fully understood. I have performed japa to my heart's content, yet I have not up to this time understood what tapas is. Hence, have I sought refuge at thy feet. Pray enlighten me about the nature of tapas."
A Guru's guidance is sine qua non in mantra's recitation, which results in BLISS.