Ok I got tired of reading like halfway down the first page so i skipped to the end to see what I could say;
For starters Master Chief is around 40-42 years old at the end of Halo 3. Second there is no way that the Halo 3 Legendary ending is Reach, because it was glassed and basically destroyed of all life via, well you all know, the Covenant. Which brings me to my next point, Reach was destroyed I'd say probably a day before Halo 1, and I'll say it took that long because of the slip space jump that The Pillar of Autumn took. Does anyone know where they jumped from? Well I do, they jumped from Reach. Does anyone know why? Because the Covenant were glassing it to hell. As I was saying the Autumn took a somewhat random jump ( i say somewhat random because Cortana had the co-ordinates she just didn't know where it led to, which of course was the first Halo and is where you began Halo 1 just after the jump). And then, if memory serves my correctly halo 1's campaign takes about 72- hours to complete, I know this because of Master Chief's mission timer in the book Halo : Flood. Then another book happens, lol, and they get to the game Halo 2 which I'd say happens about a month after Halo 1 ( don't hold me on the time frame but I know it wasn't more than six months), and then Halo 2 campaign probably takes 1-2 weeks from start to finish, because of all the traveling and what not, and then the time frame between halo 2 and 3 is about 6 weeks, to which if i'm not mistaken is when Halo ODST takes place after, and then Halo 3 probably takes less than a week to complete, so therefore the Fall of Reach and halo 3's ending probably take place anywhere from 6-8 months, I've I reread the books and played through the games I could probably give you the exact amount of months but I have final exams this week and next week. Any questions?