Orochimaru had to wait three years before transferring himself to Sasuke's body. While in the Akatsuki meeting at the end of the timeskip, they mentioned that they only had three years as well. Since we know what "thier" goal is now, one can assume that there is no internal reason for this time limit. I thought that it might have had something to do with the King of Hell Statue only being able to hold the bijuu for a period of time, but that wouldn't mean much. So after seeing Sasuke in the VoTE fight, I think Akatsuki really considers the possesion over an Uchiha...no, the Sharingan as a threat to them. Oro and Akatsuki might currently be in some race towards thier goals. Seeing as how the sharingan is appearantly connected to the bijuu(or maybe just Kyuubi i'll accept, since we don't know for sure), Orochimaru's mastery over it could quite possibly screw Akatsuki over. He is thier enemy afterall. For those saying, "So why don't they just kill him now and get rid of thier time limit?". Well, they have no idea where he is. Sasori and Kabuto's meeting ground was quite far away from Oro's HQ. Not to mention that he constantly moves around, so there's no time to waste looking for him when they should focus on obtaining all the bijuu first. This is quite possibly the reason Akatsuki has a time limit. This may have been obvious, so I'm sorry if I'm only realizing this now.