I understand what you mean, when you made the light change from 18-6 to 12-12 you just turned the timer over to the new settings is my guess, so it would take a week and you wouldn't really see anything change which more than likely had you digging up photos wondering "Why isn't my plant looking this advanced yet?". By Week-2 I'm guessing you start some of the plants start to change a little bit more, more like you expected to see in Week-1 and then now you are in Week-3 you are starting to panic because you are only seeing the buds starting to form and not everything else you are looking up in photos of the Week-1 to Week-8 time table. First don't worry, plants are not really set on a time table like you may be thinking it is... Sometimes a plant will take more time than you expect it to, as environmental factors can influence how fast or slow things change, so can things like CO2 levels. When you hear "its an 8-Week Plant" that is the average time, not a set your calender to this date and then harvest. Think of it like cooking, just because it takes the food will take 30-mins doesn't always mean the food will be ready in the 30-mins they claimed. I hope that helps.
The site which hosts this Forum is loaded with tons of very in depth information, the best thing you can do is read and learn all you can. I was just reading recently here how different strains of cannabis can be more milky or amber looking when they are done based off the strain or type more so than what we commonly hear people using as the general rule of thumb. Your growing environment from the day/night temp and humidity, how powerful your lights are, whether or not you are using CO2, it all plays a part in how fast your plants can change and grow. When you look at the professional photos keep in mind those are grown under optimal conditions by expert growers thus you will see more drastic changes in their photos than you will experience in a closet grow.
