How long does it take to develop one pointed concentration?

Hello

I was reading Om Swami's book on meditation and he stresses the importance of one pointed concentration. I have tried this method the last few days where I mentally meditate on my mantra and It seems like it is way more effective than just verbally chanting like a parrot.

I can only do it for five minute sessions at a time. I feel somewhat drained after doing it tho.

Howl long does it take to develop one pointed concentration to the point where I could do half hour sessions without much fatigue?