Perhaps you could take an official practice test, under timed conditions, and see how close you get to your target score. From what I've read at another forum: if you're within 10-12 points of your target score, then 3 months of daily practice may be enough. If you're farther off, you may need some more time. Some folks on there insist that improvements of up to 25-30 points are possible for some people, given enough practice. They also seem to be in general agreement that the 'PowerScore' line of prep books is the way to go.