That is too long. On my i7-3630QM laptop it takes about six hours or less. I haven't seen KDE (with almost all use flags enabled) take 30 hours since 2006 on my Pentium 4 and I have emerged every major and minor update since then. Even on Pentium 4 I think the usual emerge duration was around 18 hours most of the time.
I think I maybe counting KDE's dependencies into that figure. Not just KDE alone.
Interesting. I'm running a mix of i3wm and some Gnome apps on a relatively old Intel P8700 powered laptop and the only compile pain points are browsers and libreoffice (I ended up using the -bin package).