Using 1 tea spoon of sugar in a large mouthed quart jar with one packet of yeast, gives me about 3 hours of CO2 production. Since I'm always around my plants and their enclosed, I just use an open top jar in my enclosure. Definite results, greener leaves and faster growth. Also you can revitalize you yeast with more sugar after the 3 hours, just add another teaspoon. If you go with an un-lidded option you're going to have to check in on it every hour or so to watch for over flow. Another good point for the lid and tube, but their not necessary. IMHO. By using more sugar and less yeast you can get a slower "cook" that lasts longer, with less chance of bubble over.