Sustaining is used in Engineering to mean that it's now post-GA and there is no further development. The platform is not End of Life but there are no more features planned.
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.