6 months? Really? Sorry, but I totally disagree. If it is not working after 30 days of honestly trying, you would ask me to keep screwing myself up for another 5 months? In that case I would be worse off than I was before and then really lack direction or worse, be done with it completely.
I think the time period you give may be for you, or is your opinion of time needed. That is fine and I understand your point, but 6 months to you and 30 days to me is our own respective opinion, mine just happens to be based on knowing myself.
Depends on what you are trying to change. Stroke changes take a very long time to become second nature. I've heard of century break standard snooker players trying for years to add a pause into their stroke. I started working on adding a pause into my own stroke at least three years ago and am only now becoming comfortable with it, though suffice to say I have been lazy about it over the years.
But the more drastic the change, the longer it will take. Nevertheless it's always a good idea to look at your mechanics to see if there is anything that can be adjusted or even completely overhauled. Efren Reyes from the mid 80's had a completely different stance than he does now, Neils Fejien didn't always have a pronounced pause in his stroke, neither did Stephen Hendry, Ronnie O'Sullivan used to have a jab stroke (now he has the smoothest stroke of all of them). All of these players were champion caliber before making these adjustments.