Here's my crazy thinking.
Bought a bike with 27" wheels but I have a new set of 700's I'd like to put on it. The problem is the rear 700 is single speed. What I'd like to do is put the 700 rim on the rear 27" hub.
Reason:
I already have all of the parts probably minus the spokes
I've wanted to build a wheel for a while
Save money?

I realize that:
I need to make sure the frame/brakes can accommodate the wheels
Many of you will give me a hard time

I'm prepared to be talked out of this.

Really the question I'm asking is how long I should plan for this to take. I'd only have the one bike so I don't want to be out of commission for too long.