Well the series starts when the characters are 13/14, ends 11-12 books later when they are 23/24. So that's 10 years of the timeline just there, and then for my own personal satisfaction because i one day hope to do a "spin off series" on there kids, i went years past the first series ended to include marriages, house buying, pregnancies kids births and such.
Have everything all planed out completely, even the books in the series i haven't written yet and the million and one important, funny, stupid and boring events in between! Even wrote the really funny pregnancy discovery of two of the characters after the series is finished. I hope I can add the story somewhere in the Second Generation because its really really funny!
And it looks like you have a pretty massive time line to figure out yourself! Or at least a pretty big backstory. Personally i love figuring out back story, although i hate not sharing it in the books... like the funny or tragic but completely irrelevant parts.