Shaq usually retires after the first season, although he sticks around sometimes too. I've seen Garnett and Allen both retire after the first year too, but they usually take anywhere from two to four years to hang it up. Piece doesn't retire after the first two years, I don't think. He's probably good to finish out the rest of his contract.
I'm planning on starting a Boston association after the patch hits and I'm planning on keeping the Big Three together. I already have my star of the future in Rondo and Perk is a perfectly good center. Guys like Avery Bradley, Nate Robinson, and Big Baby are pretty good trade candidates. I've tried this out already, what my plan is is to trade Nate right away for Terrance Williams, who will come off the bench for a year or two while I use shooting drills to make him into an eventual replacement for Pierce or Ray. To replace Nate I'll sign Allen Iverson to a two year deal.
After the first season I'm going to shop around Avery Bradley on draft day. He's got just enough potential that I can usually package him and my own first rounder to move into the middle of the lottery, maybe higher if I package Big Baby (I'll probably keep him if Shaq retires though). Ideally this is where I would pick up an athletic big man to be my eventual replacement for KG, but I'd also like to add a three point specialist to play next to Rondo in the future. I may also look into trading Iverson at this point if I can't find a cheap and reliable replacement for Avery Bradley and Delonte West, who is usually too expensive to resign.