Ever? I know that England508 on instagram has posted how long certainpieces take, and I have seen it other places.
To speak to your question though from an outsider's perspective, the amount of time spent on a piece, especially large ones, could be seen as a private matter between them and the client. They spend a lot of time and energy on certain pieces, and are already sharing the photos. If they wanted to volunteer the info, then cool, but some might not see it as the casual onlooker's business.
That's just my ignorant perspective though.