Ahh that usually only happens in my experiences with testing/unstable packages. Generally the only time I am emerging something that takes several hours, it is a stable package that is part of my base system. X11/Qt/GTK/etc. all in one. I then start on unstable etc. but usually if those take more than 15 minutes something is very wrong

Course that was all on single core, now that I think everything in my house is at least dual-core or better it will likely only improve.
Seablade