Though this might seem like necroposting it is still quite relevant and a problem I ran in to myself recently with the current stable release of Debian. My 1 terabyte drive took a day and a half to wipe.
In Debian 7 (Wheezy) when setup says it is "erasing the disk" - the part that takes so long - you can safely hit cancel to proceed to the next step which is selecting your password. Other "cancel buttons" in the Debian setup process typically cancel the entire installation, but not this one. I have not tested this on any versions other than Wheezy.
NOTE: this of course means that you are skipping the step that writes random data over the disk, and defeating the security that provides. Frankly, if you are not concerned about the privacy of that data then by all means just cancel the wipe.