The tops should start yellowing round about 2 week into flowering,you should also start to see small white hairs but don't worry if there not there yet they will come.The flowering stage takes around 8 week but i always let mine go 1 extra week for extra crstal production.Make sure that you flush the plants about once every 2 week for best results if you are new to growing get into the flushing the plants will pay you back 10 fold.Flush with about 3 ltr of water ph of 5.5 and leave for about 1 hour in a bath so the excess water can drain off.Then after waiting add some new made nutrients when the run of has stoped leave for a while then put back in the grow room.My advice to you would be watch the i grow cronic dvd on you tube and pick some of his tips up.His yeild is not up to much to say he is growing hydrophonicly but a great watch.