ok it is a 40gb limit.
They have it as part of their fair usage policy.
40gb should not be a problem for pretty much everyone, and if you do get even close to this level of downloads then you will very quickly need to be investing in either another hard drive or lots of blank CD's or DVD's as 2 or 3 months of use at that level would fill most peoples hard drives right up
From AOL's site regarding fair usage they say
An excessive online behaviour, not representative of a normal residential use, could be for example downloading over 12,000 music tracks, or 30,000 high-quality digital photos, or around 60 movies per month. This is a very large amount by any normal standard of usage.