There is some variance depending on temperature, oxygen, aeration method and such, but high krausen usually occurs around the 12-18 hour area. I often try to time my starters to pitch them somewhere in 18-24 hour area if I am going to pitch during peak activity and not decant just to give it a bit extra time in case it starts slow. 24 hrs is a pretty good amount of time to catch the starter at or near peak activity.