How long does IELTS preparation actually take?
It depends far more on where you are starting than on how many hours you put in.
The honest answer is that it depends on the gap between the band you have and the band you need, and that gap is not obvious until you sit a full practice test.
As a rough guide, moving up half a band is usually a matter of weeks if the problem is exam technique — knowing the question types, managing the clock, understanding what the writing task is actually asking for. Moving up a full band is a matter of months, because that is a change in your English, not in your familiarity with the paper.
This is why sitting a timed practice test first is worth more than any amount of planning. It tells you which of the two problems you have.
A few things that consistently cost marks and are quick to fix: not reading the writing question carefully enough and answering a slightly different one; running out of time in Reading because of one hard passage; and treating the Speaking test as a test of vocabulary rather than of fluency.
One thing that is slow to fix and worth starting early: writing. It is the section most candidates score lowest in, and it improves through being marked and corrected, not through reading about it.