The CPAN Faq talks about mirroring CPAN via rsync.
Rsync can be very fast (after the first download
at least), but I've never used it with CPAN so YMMV.
See http://www.cpan.org/misc/cpan-faq.html#What_do_I_need
we're mirroring CPAN via rsync here too. As merlyn already pointed out, you need at least 1,3GB of free space NOW. I'd recomend at least 2.0GB as CPAN is growing and you need some overhead for logs etc.
arkab:/space/scrap/mirrors # du -s cpan
1290032 cpan
As for the time one needs for rsyncing: It's very fast.
With a 2MBit line you need about 70minutes for the initial fetch. The updates are rsynced in about 5 minutes (or less) each.