Stir well, and set it in a warm place for several hours. Watch for Froth and and Sniff. When your sponge is bubbly and has a white froth, and it smells a little sour, it is ready. The longer you let the sponge sit, the more sour flavor you will get. The proofing-time varies.
Some starters can proof up to frothiness in an hour or two.
Just experiment and see how long yours takes. If you're going to bake in the morning, set your sponge out to proof overnight. Of course, there are a lot of recipes for sourdough bread. There are also recipes for sourdough rolls, sourdough pancakes, sourdough pretzels, sourdough bagels, and probably sourdough saltines for all I know. This is the basic recipe I use, though, and it's simple and makes a fine bread.