As you may know, chanting mantras reduces karma via the chakras by sound. We should not approach japa/chanting with the impatience of the modern age.
If you have a particular goal, choose the mantra that will assist you, write out your intention and put it on your personal altar, then begin chanting it at least 108 repetitions daily for 40 days. If you skip a day, that is a signal of your inner resistance to change. Start again at day one and complete the 40 days, even if you have to do that several times.
Depending on your level of karma in the area of your intention, you may get what you are chanting for in one 40-day sadhana or have to do it more times.
One of my late husband’s students wanted material success in his business. He decided to do a sadhana/discipline of 125,000 repetitions of Lakshmi’s moola mantra. When he did not get prosperity on completion, he did another 125,000 repetitions. He still did not reach his goal. He knew it was his karma, so he did a third 125,000 repetitions and then reported to Namadeva Acharya (Thomas Ashley-Farrand) that he had so much success that he did not know what to do with it all.
To succeed in chanting mantra for any goal we need to have patience and tenacity.