There seem to be an awful lot of variables in determining time to 1st solo (ability of student, ability of instructor, application by student, consistancy of lessons, airfield environment etc etc).
However, maybe the greatest determinant of all is aircraft type. Like it or not, with all other things being equal, a student is going to solo quicker in an S300 than in the R22.
I would hazard a guess that the 20hr limit applies to the R22, and it's about right.
But (as I hope Whirlybird will back me up), please try not to get too hung up about how long it takes to go first solo. On the grand scheme of things, it's really not that important. Different people excel at different things. A student who is good at the 'aviate' side of things (ie time to first solo) might well struggle with the 'communicate' and/or 'navigate' side of things. The only people that 'time to first solo' really matters to seem to be those who are 'my car is faster than yours' or 'my w*lly is bigger than yours' types.
Hope this helps,
B73