0.15 supports fee estimates up to 1008 blocks
It would be great if someone could add support for this to the GUI with a redesign of the slider.
My recommendation was just to make the slider have discrete values such as:
2, 4, 6, 12, 24, 48, 144, 504, 1008

Do we really need more fee choices than that?
But perhaps someone with more QT expertise could whip it up faster than me