I use two tablespoons per cup of water, with instants it might be hit and miss, so play around with dose until you find what you enjoy. If I remember right BKH recommends two teaspoons per cup, so go from there and work your way around. Have you ever drank kava before? If not, then I wouldn't anticipate too much of an effect your first time. It seems to be the general consensus among kava drinkers that at first, it's either a very light effect or nothing. Over time, though, a sort of reverse tolerance goes into effect, in which you will begin to enjoy kava for all it's worth. Like always, your mileage will indeed vary.
I've never had a kava experience last 5 hours. if it's potent enough kava to last 5 hours, I usually pass out. lol. Kava is best used, at least in my personal opinion, after a stressful day, unwinding for a few hours. Most people I know don't enjoy using kava while under particularly stressful work situations, but we work concrete so I suppose it depends on your job. I just wouldn't expect it to last 5 hours, maybe 2 - 3 hours at most after reverse tolerance.
How long 100g lasts depends on how much kava you find you like per serving. I'm almost out of my tin of instant as I use 2 - 3 tbsp per cup of water. It arrived this last Thursday, and now it's Monday night, and I've still got about 5 good servings (based on my prep method) left.
Hope that's some questions. Feel free to ask as much as you wish, we've got a good tight knit community of kava drinkers here.