How Many Hours Does It Take To Move Up One IELTS Band Score?

(TESOL Ideas--IELTS)

There's been some internal debate at work about how many hours a student needs to study to move up one IELTS band. (On average--I know the individual variables are potentially infinite.  But we wanted to find out an average estimate for the purposes of deciding the length of our courses.)

As part of that debate, I spent some time researching on Google, and came up with some quotes from a few websites.
It occurred to me the other day that I should keep this research handy in case I ever need to reference it again.  So rather than have it get lost in my email archives, I decided I would post it on this blog.

Below is what I found out:

Source 1:
“The research revealed that around 300 hours of full-time study (18 hours or more a week) is needed to lift the average candidate’s overall band score from 5.5 to 6.0. Surprisingly, longer than 300 hours of full-time study is needed to move from 6.0 to 6.5, or from 6.5 to 7.0.”
Source: Ryan's IELTS Blog http://ieltsielts.com/more/study-plans/

Source 2:
In the past, IELTS released a study that suggested it takes around 200 hours of course study for a person to improve their band score by 1. Our personal experience combined with this research verify this.  In other words, a student should study around 5 hours/day for 2 months to improve his/her band score by 1.
Source: Scott's English Success http://answers.scottsenglish.com/discussion/264/how-long-does-it-take-to-improve-your-ielts-score-by-1-band

And
Source 3:
There have been a number of studies investigating the relationship between English language tuition and scores on tests of academic English. However, those studies which have investigated gain scores on IELTS following IELTS preparation courses have produced somewhat mixed results, possibly due to variations in sample size and course duration. For example, in a small-scale (n=17) study in New Zealand, Read and Hays (2003) found that gains made by the students following one month of an IELTS preparation course were not statistically significant. In a large-scale study (n=476) Green and Weir (2003) found that, on average, students’ scores only increased by 0.21 of a band (from an average of score of 5.27 to 5.48) following 3-12 weeks of intensive IELTS preparation and English for Academic Purposes type courses. Retrospective studies based on results from candidates who had taken IELTS on more than one occasion over varying intervals of time (Gardiner 1999, Green and Weir 2002, 2003) produced similar results.
In contrast, Elder and O’Loughlin (2003) found that 10-12 weeks of intensive English language courses in Australia and New Zealand (n=112) resulted in a significant improvement in English language proficiency, with students on average increasing their IELTS score by half a band. They found that the improvement was greatest on the Listening subtest, and gains were likely to be greater for students with low initial ELP. They also found that a range of personal, instructional and environmental factors were linked to these improved scores, but that these factors varied from one language skill to another. However, Elder and O’Loughlin (2003) suggest the need for caution in interpreting these results, as these studies are measuring average gains, and individual performance is highly variable.
Source: WHAT HAPPENS TO INTERNATIONAL STUDENTS’ ENGLISH AFTER ONE SEMESTER AT UNIVERSITY? http://www.jbe-platform.com/content/journals/10.2104/aral0804?crawler=true

*END QUOTES*

(The first source I had already used on a previous post.)

One of my colleagues pointed out to me that of these 3 sources, the first 2 were just blogs, and couldn't be used as serious sources.
Both of the first 2 sources claim to be based on serious studies, but do not link to their respective sources.
I, however, tend towards credulity, and am prone to assuming that even though these blogs didn't accurately cite their sources, they probably are based off of real studies nonetheless.  Use your own judgement I guess.
The 3rd source is reputable.  (In fact the author Neomy Storch is someone I knew about, and had some brief interactions with, during my time at the University of Melbourne.)

But I want to open this up to comment and discussion.  Please comment and let me know what you think.  Let me know of any other studies you are aware of.  Anecdotal evidence and personal experience is also welcome.