I've been baking for a number of years, and am still learning. I have a rather simple question (which may have a complex answer). The question is: How much yeast should I use?

Yeast is, of course, a living thing, and will reproduce so long as food is available, and conditions are suitable (no organism can live in an environment of its own waste products.) This being so, it would seem that the major variable in leavening dough is time. A small amount of yeast, over a longer time should result in the same effect as a larger amount of initial yeast would produce in a shorter time. Why then do different recipes call for different amounts of yeast?
Tom

Someone just gifted me with a bread book called "The New Bread Book" by Ursula Ferrigno, which I do really appreciate, but all the recipes call for FRESH YEAST, such as 1/4 oz(10G) crumbled, is there some sort of a conversion to instant or rapid rise yeast?  I really do not want to search out a bakery, not any in my loacal area, to get some, and I've read it does not last very long, any ideas?  Thanks. Russ