Shouldn't take that long. You'd just want to set a date and get the invites out. All of the baby showers I've been to, including mine have just been an afternoon tea type thing. Starting around 2pm finishing up around 4-5pm. Lots of finger foods, cakes, slices, nibblies, tea, coffee, soft drinks. A couple of games (there's heaps on the net you can google). Opening presents. A few balloons stuck up around the place.