I have watched this thread with great interest.
Done my fair share share of ab initio instructing, 1000 hours, getting on to 35 years ago.
One of the greatest joys I got from the job was sending a person on their first solo.
I got some "away" in 5 hours but it is risky, thinking back. It only takes something out of the ordinary to occur and there could be trouble. Others took quiet a bit longer, various reasons already discussed on this thread.
The hardest to try and teach were the young bucks who had lessons from a number of training establishments, or a friend or relative had taught them a bit, or had grown up around aircraft.. I.E., preconceived ideas which in most cases were hard to get around.
The easiest to teach were the young ladies of 16 or so, who had no preconceived ideas and would do what was asked.. In fact some couldn't even open the a/c door until well into the 2nd or 3rd leeson.
When I go back through a pilots logbook now and see that as a student they did some advanced exercises like steep turns etc., before going solo then I know they have had probably a good instructor.
The best environment to get someone away quick, is a non controlled airfield, no other traffic, grass and into wind. Do 1 lesson the first day and 2 lessens each day after but of course that is rare these days.