Using a small funnel with a wet tobacco, I can get close to 4 hours of smoking time if I am careful. Of course, that is too long of a period for smoking, so I usually end up quitting after about 2 hours so. Like Sambooka said, after about 2 hours, there is quite a bit of unburnt tobacco left at the bottom of the funnel. So I scrape off the burnt layer of tobacco carefully and keep the rest in the funnel. When the next smoking session rolls around, I simply fill the top of the funnel with fresh tobacco and keep smoking. I have three funnel bowls and I usually have 3 or 4 different kinds of tobacco open at a time so this setup works for me.
I have asked a friend of mine to prepare multiple bowls, some with left over tobacco at the bottom and some entirely fresh, and did a blind taste test, and I wasn't able to discern any differences in the taste, therefore this setup works very well for me. Of course, people with a more sensitive palate might have issues with the left over tobacco...