Someone's old data from 2012 to build a core-image-sato using Yocto is just over an hour - http://www.burtonini.com/blog/2012/11/15/yocto-build-times/.

That said, core-image-sato used to be just a simple demo image. In my old build system, we would build the Arago Project for a TI SoC every night and it would take a few hours but that included a lot of the DSP code as well which was really slow too. So a couple of hours on average is my guess.