How long should it take to enable FileVault 2 on a fresh Lion installation?

Question

How long should I expect it to take to enable FileVault 2 on a fresh installation of Lion? I'm using a mid-2010 15" MacBook Pro with an i7 and a 5,400RPM 500GB hard drive with only 10GB used.

John Siracusa's 19-page review for Ars Technica had this to say:

Encryption happens transparently in the background, which is a good thing because it takes a long time.

Hopefully someone can be a bit more precise than this.