Make it as long as you want mate, just keep an eye on the file size when you go ahead and record. a good idea is to change the split to 2048 in the recording preferences, which gives you about 2 1/2 hours before the file splits.
personally 1 hour + is what I aim for, make sure the file size on Traktor is set to higher than 700mb as well, then start your mix if you do decide on an hour or more you can easily import it to Itunes, then covert to AAC or MP3 it will easily fit on a disk, converting it will shrink the file down to about 130-150mb