Depends on where you cut it. I think in my pics I show where to cut it. I tried to cut it large, then cut down to fit. I had to pull most of the dash, (which I was going to do later, not now) and pull the heater to access where I wanted to cut. If you cut (in the front) just below the pinch welded seam (on the hump you are using), it will fit good.
Look thru the pics in my cab swap thread, and if you have more questions, fire away.
Oh, I didn't answer the time question.

It can be done in a weekend, if you don't screw off much. I think I had about 6 hours in all cutting the humps out and fitting my hump to the donor cab.
This isn't counting the dash and heater BS.