I recently graduated from one. It was 12 weeks, 8 hours a day. The school I went with is supposedly one of the higher rated, but there's really a million to choose from. The program you choose matters a lot.
I definitely learned more, faster, by being in a class and having actual instructors, but I also learned more in the month after on my own, than I did in the three months. It was a lot of repetition of basics but it's hard to get a full understanding of software engineering or any computer science basics in 12 weeks. I'm still looking for a job, and bombed an assessment recently for a company, but it's only been 2 months. I've seen a lot of my classmates starting to get hired.