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It's ready when it's lively and bubbly, and it doubles quickly. It will get better with age, but there's no reason you can't use a young starter to make good bread. My starters are all over 10 years old at this point, and they have changed since they were "new."

If you've got a recipe that calls for a biga, poolish, or sponge to be made ahead of time, you could use some of your starter in its place. You'd need to figure out how much starter and how thick or thin it needs to be. After that, you'd need to rely on the bread's response rather than time, because sourdough can be a little quirky as far as how fast it rises.

If you need to follow a recipe exactly, I'd suggest going with one written specifically for sourdough to make it easier, though.

@mollykate678- welcome to the world of sourdough. I have just recently gotten into sourdoughs, and I know how confusing it can be. You can get totally confused and misinformed if you read everything on the web. Even some very good books have confusing and contradicting opionions on sourdough.Biga, poolish,sponge,starter,barm,mother culture,seed culture etc. all some of the terms you will see, and sometimes they mean different things to different book authors. My advice would be to work with one source of info at a time until you feel comfortable with the results. I am doing a lot of work with Peter Reinhart's "THE BREAD BAKER'S APPRETICE", and I have had very good results. One word of caution, some of his terminology can get a little confusing, and In a newer book of his on Whole Grains he admits as so. As for substituting sourdough starter for dry yeast, of course you can, thats what sourdough bread is, a bread leavened with a sourdough culture rather than with yeast. I have been successful converting a yeast leavened recipe to a sourdough recipe by using 1/4 cup of sourdough starter in place if 1/4teaspoon of yeast. If a recipe called for more yeast, I would adjust accordingly. I have used this with a no knead recipe which calls for 3 cups of flour, so obviously this is not an overly large loaf. Also, the no knead recipe calls for a long (18 hours) ferment at room temperature, so the sourdough starter has plenty of time to do it's voodoo. If you would like a good source of info on this recipe, go to "breadtopia.com",some great info there. Good luck.Just experiment and keep notes, that part of the fun. Longer fermentation time can improve your results greatly, and if you get good with your sourdough culture, you won't need to use yeast anymore. I now make all my pizza doughs with a sourdough starter and they are delicious.

Well then... pass on a good way to begin a starter. I just made a rye starter that in it's initial week of bubbling went alcoholic and disgusting.

@Elizabelle- google "breadtopia.com", follow his instructions for starting a starter using the "Pineapple Solution" method. You will need some pineapple juice to begin. Just a can of canned pineapple in it's own natural juice is all you need. The instructions are very simple and results are almost guaranteed. The pineapple juice provides an acidic atmosphere which keeps the starter free of the bacteria (leuconostoc) which is responcible for the problem you had. Rye flour has been described as a "zoo" of organisms, so it is easily subject to the bacteria. I started a sourdough rye with this method just 2 weeks ago, and it is excellent. Good luck! Be sure to stir your starter often during the first 3 days.

@mollykate678- don't forget your public library, I have a copy of " The Bread Baker's Appretice" right now, that a borrowed from the library. They even let me renew it for a second month. Just about all the books that people on this site will recommend,are available through public libraries. Don't let the sourdough info overhelm you, it's not all as complicated as some people make it out to be. Breadtopia's info on starting a starter should put you well on your way. Good luck.