The time it takes a starter to show signs of activity is related to the amount of yeast you add and the volume of your starter. Thus, for example, I make a starter from the large Wyeast smack packs sand add it to about 2 1/2 cups of sterile wort. That is, I think, about 25 billion cells into this volume. I see activity in about 12 hours. I often make my starters on Thursday night for Saturday brew.
Dr Malt