Here are some preliminary numbers. Final numbers will probably be a
bit different, but those should be a reasonable first-order
approximation. Of course, your mileage may vary.
-6GB of download.
-25GB disk space to do a single build.
-80GB disk space to build all AOSP configs at the same time.
-16GB RAM recommended, more preferred, anything less will measurably
benefit from using an SSD.
-5+ hours of CPU time for a single build, 25+ minutes of wall time, as
measured on my workstation (dual-E5620 i.e. 2x quad-core 2.4GHz HT,
with 24GB of RAM, no SSD),
Naturally, if you plan to work on multiple branches (e.g. ICS release,
ICS development branch and AOSP master branch), you need to plan for
disk space accordingly, and you could easily end up using 250+GB.