ok in that case now that I know what setting you are talking about,
yes, that does seem a BIT too high,
I'd recommend anywhere between 2-5 % of the size of the drive depending on the total size of the drive
It all depends on what you do online, whether it's just casual browsing or forum perusing all the way to Flash heavy sites or audio/video embedded pages.
Also, your internet connection may help you determine what size the cache could/should be, a larger cache could help someone with dial-up, as they could cache more of their favorite site's files and not have to wait as long while they wait for the content to be re-downloaded everytime they visit that page.
The only worry is that sometimes too much can, yes steal from your available drive space and make cleanup take longer , but in today's day and age of 500 GB hard drives becoming publicly available, and relatively cheap(ahem, sorta) drive space isnt as big of a deal as it used to be.
And I'm sure someone else may have something to say about it, but yes, 30 GB is a little too hefty for the IE cache.