Many recipes at Sourdoughhome call for Active Sourdough Starter - so, what's an active sourdough starter?
An active starter is one that has been fed within the past 12 hours, and that is active enough that it was able to double in size after that feeding.
If you fed your starter and it didn't double, you should feed it a few more times. I also suggest you check out our page on maintaining or caring for your starter.
The best time to use the starter is somewhere between the time it reaches its peak and before it starts to fall. When that will be depends on your starter and the temperature at which it is being stored.
If you keep your starter thinner (that is, you feed it more water and less flour) than I suggest, it may not double at all. Thinner starters don't have enough gluten in them to rise. Thinner starters also work more quickly and can eat all the food in the starter and get into trouble if you neglect them. For many reasons, I don't suggest thinner starters for beginning sourdough bakers.