What seems to vary from one school to another is how much of the syllabus has to be taught & proficiency demonstrated before the magic of first solo is allowed.
At Cabair I (correctly in my opinion) had to learn and demonstrate EFATO, etc. etc. before being allowed to go solo.
Oh. And I took some time to master the flare, so I was a shade over 16 hours before mine.
3 things I remember - how quickly the 172 climbed without an instructor, no leg in the way to get to flap control, and total, utter euphoria!!