by jesse
Just read about this in the new Gentoo Linux Newsletter:

One can use genlop (0.30.7+) to estimate the time of an upcoming compile job now even if if a package has never been compiled on the system before. Just add the -q option which makes genlop query a database running on gentoo.linuxhowtos.org for compile times. The data is based on users with identical CPUs and it’s quite accurate since it calculates the average time.

Just give it a try with, e.g.: $ emerge -uDNpv world | genlop -p -q

If you didn’t know about genlop, emerge it with $ emerge -av app-portage/genlop

I usually use genlop to get the estimated time left when running a system update.

$ while genlop -c; do sleep 5; done

gives an output like

Currently merging 1 out of 6

* sys-libs/pam-0.99.8.0-r2

current merge time: 24 seconds.
ETA: 1 minute and 24 seconds.

Have fun :)