It's not like it's guaranteed to happen at a certain time. It's just because the Xbox 360 was poorly made and it's very common for it to suffer from hardware failures. Hell, you'd be lucky, yet it's possible, to have your 360 working forever without suffering the RRoD.
My 360 had it before my brother gave it to me, then when he bought a new one I got his old one fixed, and I've been playing almost non-stop since I got it fixed two months ago, with no problems.
I have, however suffered from the RRoD once since it was fixed, but it worked immediately after and has worked fine since.