0-2 weeks - the plant notices the change from 18/6 or 24/0 to 12/12. It rearranges it's priorities, and so it's metabolism in order to start bud production. The plant will continue to grow in height.
2-3 weeks - the first pistils or balls appear. The very first will show at the main nodes (between the main stem and a secondary stem). Males tend to show sex earlier than females since males have a faster metabolism.
3-5 weeks - pistils have formed on all nodes or stem tips.
5-6/8 weeks (depending on strain) - the actual buds form.
6/8-9/12 weeks (depending on strain) - the buds show a higher density.
7/9-10/12 (depending on strain) - leaves start to yellow as a sign that N once stored is being used for bud production. The plant shows a condition known as Sinsemilla (sin - without, semilla - seeds, both spanish terms). In this condition, fake seed pouches begin to form, the plant pretending that it is pollinated. During this period, bud size doubles.
During flowering, but never on a fixed schedule, the pistils will turn from white to red/amber/copper/brown and the trichomes will turn from clear to milky and then amber. It is believed to be ideal for pistils to be over 70% red when harvesting.
Also, clear trichomes have no effect, milky trichomes produce a head buzz, typical for sativas, red trichomes produce a body stone, typical for indicas. In my experience, sativas will naturally present more milky than red trichomes while indicas more red trichomes. Depending on the type of high you are looking for, wait for the milky/red ratio you are looking for and then harvest.
Flush when the 25-30% of pistils start turning red, so you know that you have a minimum of 2 weeks flushing time.