Depending upon which one you pick first—you have three elements now, with e-mails, data centres, and the network—the easiest one would probably be e-mails in terms of sequencing. The hardest one would probably be the network. The data centre would be somewhere in the middle.
The data centre consolidation really depends upon what is classified as a data centre today. I'm almost afraid to ask the question here. In government, the data centre could be a couple of servers sitting in the closet, or it could be a data centre that is properly breathing and everything else, the way a data centre is. Depending on what you have, on what you find in your investigation process, on what the team finds, they would know best.
So to come up with a number saying that all the consolidation should be done by this time would be a hard thing to do, because I don't know what you have.