For about 4 years now, I keep my MAME ROMs and CHDs up-to-date (complete). I think it's at about 80Gb. I'm not at home right now, so I can't check.
I also collect the MAME - Rollback ROMs, so I can always go back to an older MAME version if I want. For anyone who wants it, the dat can be found
here (no ROMs, just dats here).
A small remark: whatever people say, you CAN'T keep your MAME up-to-date without a decent RomManager like clrmame or RomCenter.