@DUGG: don't mind the guys, they're just playing with each other...
We can help you with making recordings of TV shows for your own use :iagree:, just not the thing about copying rental DVDs :disagree:.
Most recording software will allow you to select the quality, and thus the amount of time used on the disc, but I have to say that 6 hours of video on a DVD will look pretty bad. Recordable DVD discs are so cheap that it's really not worth trying to cram too much onto them.
I have used DVD Movie Factory 4 Disc Creator to record digital transmissions from a DVB-T capture card, and it works very well. You can do the recordings direct to disc if you want, rather than to the hard drive first. DVDMF4-DC has editing facilities to cut out adverts and stuff, and is an authoring app as well to burn the edited video to DVD.
If you're wanting to record analogue TV, the you'll need an analogue MPEG2 capture card (rather than the digital one) to convert the signals to digital first. As far as I know with these you have to record to hard drive, then burn to DVD later. DVDMF4-DC should also work well with these digitised recordings.