My thesis (in math / CS) was essentially several papers stapled together, except that I made the papers into chapters, expanded/rewrote bits and pieces, and added narrative to glue the chapters together.
This took about six months. In retrospect, I think this time was well spent. I developed my understanding of the material quite substantially by reworking it. But doing so was certainly not nearly as fun as writing new papers.
Same here except that for me it feels like wasting a lot of time, unlearning all my other skills and not staying up to date with current trends.
Having to rewrite polished and reviewed papers is aweful and not publishing stuff (except in a thesis) is unacceptable for many institutions, so I think making papers to chapters is the way to go as long as there isn't something completely different (e.g. no prose but making all data, hardware, software, documentation etc open source)
This took about six months. In retrospect, I think this time was well spent. I developed my understanding of the material quite substantially by reworking it. But doing so was certainly not nearly as fun as writing new papers.