But, realistically, I think it's safe to assume a ratio of anywhere between 13 to 17X. You'll probably see lower ratios on target-based deduplication, and you'll see higher ratios on source-based deduplication just because of how they are architected.
Now the flip side of that is the kind of data reduction ratios you might see in an archiving environment. I recently had a chance to speak with companies like Permabit and NEC that are seeing their appliances deployed more in that context. They're seeing ratios from as small as 2X to as large as 200X. So again, your mileage will vary; a ratio of 13X to 15X is a good rule of thumb, but you could see extraordinary results, if you are in the right kind of environment.
Check out the entire Data Deduplication FAQ.