There's no way in Hell you have a store-bought DVD that's 5-6 hours on one side, but yes, if you compile your own, you can fit that much on (albeit at a much lower bitrate than pre-recorded discs).
But yes, this practise was very common with the first batch of DVDs - there's a terrible DVD of Goodfellas that's still sold in HMV and the like, where you have to swap sides. There are a few DVDs that make semi-smart use of the double-sides, though - A Bug's Life and some of the Superman DVDs have 4:3 full frame and 16:9 widescreen on different sides, although to be honest, it's pretty redunant when most TVs can crop/stretch the picture anyway.