Yes, I should have made the point that there are part-time options, not just at "mainstream" studios but at some ateliers. Peter Bougie has a small operation and really can't afford to have a lot of part-timers occupying studio space, but I believe The Atelier (in Minneapolis, the successor to Atelier Lack) has part-time opportunities, as does the
Minnesota River School of Fine Art. In retrospect, I do wonder if that wouldn't have been as efficient and valuable an experience, though I do think the discipline of being physically at the studio for many hours each day, with nowhere else to go and no distractions, was just what I ("easily distracted") needed. The risk was that I probably would otherwise have continued to see myself as a hobbyist, and I wanted more.
You also asked about admission standards. I can't speak to anything but my own experience. I had very few drawings and paintings to show when I interviewed, though I certainly had more than a lot of applicants show up with. They didn't demonstrate that I was a good artist, but that I was teachable, that I had ability and that, by virtue of just having generated some work on my own, I was reasonably serious about a long-term course of study and determined to excel. Certainly the process isn't a "formality". An atelier's reputation rests in part on the quality of work done by its students (as exhibited annually at a student show), and there's simply nothing in it for an instructor to take on the unqualified simply to help pay the rent. It'll come back to haunt the place.
Lastly, there is no single "atelier experience." Procedures and emphases vary from studio to studio and from year to year at the same studio.