How much data can a double sided double layer DVD hold?

First off, a calrification: I am talking about a "regular" DL: disk, not a Blu-Ray disk.

The capacity printed on the disk is 8.5 Gb, but some of this goes to overheads and formatting information. The useable space is more like 8.2 Gb, and can vary depending on the data being stored, e.g. movies versus data.