This topic has been rattling around in my brain for decades. Especially in higher education, there appears to be a consensus that transcribing the improvised solos of the masters is an important part of learning to be a jazz musician. I have a confession. I am not so sure.
Let me begin by saying that I have transcribed many solos over the years. It is a practice that strengthens connections between the ears and the musical mind. It is part of the process of learning the language of jazz. When I taught undergraduate jazz improvisation, I always gave transcription assignments. I have even made calligraphy scores of a few of my favorite transcriptions. My thoughts here are more about the long-term strategies for advanced students and professional musicians.
My undergraduate experience was filled with contrasts. At the same time that I was taking improvisation classes that required transcription projects that were both written down and performed, I was studying with Yusef Lateef. Yusef was very against transcribing. He felt that the practice only made you sound like someone else, and that the whole point of being a musician was to be unique. He saw transcribing as a way to move backwards as an artist.
I have never been great at transcribing. I can do it, of course, but I'm slow and I don't enjoy getting all the details perfectly written down. I'm always second guessing how much detail to include. Articulations? Fingerings? Time feel? I find more value in playing along with the recording, at least when it comes to getting the details right, but I dread writing down things like "lay back," or "straight-ish swing eighth notes." Ugh. But I also find tremendous value in writing things down for later analysis, and for documenting the work.
In the twenty years that I have been a professor, I haven't transcribed many entire solos, except for a book that I wrote for Hal Leonard that was more of a project that I took on for professional development than passion for the subject. I have tended instead towards targeting portions of solos that I am particularly drawn to. From there, I try to reverse engineer the essence of the passage so that I can learn what made it stand out to me. I do the same thing with passages from classical music. For example, I have a page in my notebook of patterns from Astor Piazzolla's etudes that I have expanded into different keys and transpositions.
This falls inline with a practice from my lessons with Yusef. We would improvise (although he never used the word, preferring his autophysiopsychic terminology) and then go back and revisit ideas that struck us as interesting. He would point out if you happened to play something more than once, or if you developed a certain idea. He would encourage you to uncover the essence of the idea and expand upon it. It was a bit like transcribing yourself as a way of accessing your own musical subconscious. In this way, you could develop your own internal voice into a unique form of music expression.
I think that an initial period of transcribing entire solos is probably important, as it gives a tangible framework for building a set of vital skills for the improviser. Once those skills are in place, I think that it should be up to the musician. How many solos are enough? I guess it depends on the individual. I have had students that labor for weeks on one solo, and others that can pop them off in a few days, or even hours.
More importantly, if you enjoy transcribing, go at it! I enjoy hearing all the amazing work that people post on social media. I also find it very interesting to analyze written transcriptions that I might not have had the time or inclination to do on my own. For some people, transcribing is an important and rewarding part of the overall work on being a jazz musician.
My personal practice has sharpened into exploring how things work, and how I can rebuild them in my own way to make them distinctive. It's the same for me when I play Bach or Paganini. I'm not interested in being stylistic perfect or correct. I'm trying to find a unique way of performing material that is meaningful to me, knowing that if it is unique enough, I might build an audience that will recognize that they can only get that particular music by listening to me.
So, if you are a young student, definitely transcribe. Just try to see the big picture and to find a way to be true to yourself as you explore other languages and styles. Practice well!