Over the years, the recommendations have all changed, and the way things were handled with my DD is not what would be done today for the exact same issues. She had speech, OT, and physical therapy when she was little. Later she had cognitive behavioral therapy, social skills classes, and just good old talk therapy.
I do think that the various things we've done have helped a great deal, but answering "how long did therapy take" is tricky. My DD is on the spectrum. She will always be on the spectrum. There isn't a therapy that will change that.
However, she can talk and make her thoughts understandable to others, she can listen, she can read basic body language and carry on a conversation. She is comfortable in her own skin and has learned to mostly cope with the anxiety that is often co-morbid with high functioning autism. She has a life that works for her for now, and we will continue to work with her and adjust and she continues to be ready for more -- in her own time and her own pace.
She had more speech therapy than anything else, and we figured out ways to get what she needed from OT in other ways, like swimming.
But the crazy thing about raising a special needs child is that you never really know what is making a difference, or if more of X would have been better. She placed into a developmental preschool and I didn't send her, and I've sometimes wondered if she would have done better if I had. Sometimes it's just a judgement call, and in the end, we all just go with our guts and hope that what we choose will work out for the best for our babies.