Are you just trying to burn the files as data so that you can use them on your computer, or are you wanting to convert them to dvd so that they can play on a normal standalone dvd player? If you want to convert them to dvd, nero must convert the avi files to mpeg2 (which is what dvd uses) and that is what takes so long, the conversion, and as others have said, it is dependant on the speed of your computer. I can do it in around 3 hours or less for a full dvd’s worth of video on an athlon xp 2500@3200, with 1 gig pc3200. I have heard of it taking less that 2 hours on an athlon 64 system.
If you just want to burn the avi files as data to a dvd (they will not be converted to dvd and will not play in standalone players but can be read on your computer), select the make data dvd option in nero, which takes substantiall less time.
Also, what media are you using that is 2x? That is half an hour of your time right there. USe dvd identifier to check the media id on your disks.