There are two schools of thought about liquid starters. The first school says to pitch the starter at peak activity, usually 18 to 24 hours into fermentation. The yeast is still in suspension, so you have to pitch the whole thing, possibly adding some off flavors from the starter beer. The second school says to let the starter complete fermentation and rebuild its reserves of trehalose, glycogen and other nutrients. You let the starter ferment for a couple of days then refrigerate. The yeast will settle out in a day or so, and you can pour off the starter beer, leaving just enough to swirl up the yeast.
If I have time I like to do it the second way. As Yooper mentioned, that gives you a chance to build it up multiple times if you want. It also lets you pitch just yeast. If I don't have time I'll do it the first way. If I
really don't have time I'll just throw money at the problem and pitch multiple packs of yeast.
My last brew was a Vienna Lager. I realized too late (Sunday afternoon) that my only possible brew day was the next day. There was no way to build a starter so I just bought a couple of extra yeast packs and pitched three of the damn things. It's chugging along nicely but I don't want to do that again -- $20 worth of yeast adds up.
Chad
Yeast Starter Section of Palmer's "How to Brew"