LOL, if you are like me, you wait until you get halfway around World Showcase in Epcot to decide you can't move another step, and call to have someone delivery one right then... That's how I rented my first
ECV!
NO, DON'T DO THAT - seriously, even though January is slow, it's a great idea to plan ahead a bit!
You don't have to rent them *today*, but if you are like me, I *LOVE* ticking off those boxes on my "to-do" list, and renting ahead is something you can easily accomplish. I'm already knee-deep into planning for our January trip.
If you rent any time before December, I would say just check in with your rental vendor at about the 30 day mark, and then I would call and do a "reminder" check between 5 & 7 days prior to arrival. (That's just me; you don't have to do that - I have just learned that it's worth it for my peace of mind!)
January is slow, so you can probably safely wait until 30 days out