12 hours is the limit for file downloads since the member may not want the file downloaded several times over a long time period of time. We are working on upgrading the system to allow more options regarding this.
If I was a buyer with a slow internet connection and I want to download a big file, that might cause some problem. A day or a week is probably better for the minimum limit.
Still, I don't understand why a member might not want his file to be downloaded several times (assuming IP & cookie is logged for downloading)? Is it for piracy protection or something like that? Do they request this or you set it on your own?