Hmm, I built gcc 4.7 a while ago on a dual core laptop and it couldn't have taken much more than an hour, I think, if that, including the standard libraries. For sure kernel builds on a contemporary machine are like 5-10 minutes. Configuring the thing is what takes the better part of a year .
My 3.1 GHz Dual Core takes ~ 35 mins, with -j3 .
(It was an adventure, the first few times, to compile the custom kernels for my netbook on my desktop.)
(I read a benchmark article a few months ago that the best i7 processor did it in 1 min, when highly overclocked.)