So in one of the gameplay reveals for spartan ops, in the top right hand corner, there’s a timer which says “Episode 1: Time Remaining: 6 Days 14 hours”.
I hope this doesn’t mean that we’ll have a limited time to play an episode, doesn’t seem fair to people who buy the game later on. Obviously this is speculation, and I really hope they don’t carry forth this decision, if that’s what this ends up being.
Source: http://www.youtube.com/watch?v=F0DSvSif23I
Skip to 0:59