The movie length you can fit on a DVD is determined by the compression / quality used. It is possible to squeeze 8 to 10 hours of footage onto a disc but don't expect better then VHS quality at that rate.
I would add that consider your source data, if you are downloading xvid at 400mb per 45min then burning 2 or 3 episodes per DVD will not improve the quality, were as if you have say HD capture at 1.6GB per 45 min then you will lose quality putting 5 or 6 on one DVD.
Generally accepted ratio on the downloads seems to be 4 x 45min episodes per DVD giving a run time of around 3 hours.
Oneman is spot on, in that it depends. Personally, I've only got a standard CRT TV, so I can get away with 5-6 episodes per standard disc. I'm sure on a large HD TV they'd look awful, but for me they work fine. You could various combinations to see what suits you. But then again, discs are so cheap these days you may as well use the minimum episodes per disc if you're that bothered about quality.