What CPU do you have? And how much RAM?
If you are trying to burn your mpegs on to the DVD so you can watch them on a stand-alone player then they have to be converted in to DVD files (.vobs etc), this process can take a while.
18 hours sounds rather long though (unless you are using an older CPU)! I would guess that even a double layer disk should take a max of 3 hours on a modern CPU (only a guess as I've only ever done single layer disks)
If you just want the files backed up to DVD then then plain old Nero Express would be btter than Vision Express