It can be years or the whole life time!!

Concentration is impossible for ones who’s done the worst sin tho. Murder their own parents for example.

It is also hard to achieve for those who’s done too much unwholesome act because their mind will often associated with remorse thought, hence, it’s hard to train to bring their mind to stay still.

But they have chance to replace the frequency of the remorse thought arise with wholesome act which could make one feel better about their self as a human being. By knowing their self doesn’t did something harmful towards other beings, the mind will satisfied and become more benign to be directed to where one wants.