40 hours seems a bit much - there would have to be a bunch of messages moving around for it to take anywhere near that long.
You'd be using COBRAS for this:
http://www.ciscounitytools.com/App_COBRAS.htm
ideally (and I think you probably have to given the hardware requirements) you'll be installing onto a new server. If that's the case you can run the export on CUC 1.2 without it being down, run the import on the CUC 7 system and once it's done and you've verified everything is cool just redirect your call manager ports - zero down time.
If you're planning on using the same server then yes - this is going to take a while since you'll have to do the export, wipe, install Connection, configure, import COBRAS and then you're flying - that'll be a chunk of down time no matter what you do, though.
The COBRAS help file on the page above has some real world time estimates for exports an dimports on a few systems from the field - you can use that to get a very rough idea of what kinds of times you'll be looking at - there's no universal formula of course but this will get you in the ballpark based on how large your system is, how much message space is involved etc...
