I've done probably a dozen or so swaps and I will say it depends"

A basic swap, just transferring from one PF to another, isn't really difficult as long as you have some things:
1) Patience.
2) Lots of pictures
3) Manual to the game
4) Decent tools

If you take pictures, take a lot. If you think you've taken a lot, take more. Reassembly is just essentially the opposite of disassembly.
You need the right tools. I have a friend who thought they could do it with pretty much a screwdriver and pair of pliers. And the game manual usually has assembly drawings of the mechs, so instead of trying to figure out where that oddball spring goes you can easily look.

Time depends. Depends on your schedule, the game, your experience, and if you're doing a swap or a restore. AFM is one of the easiest swaps out there. I've knocked AFM swaps out in 20 hours or less. But if you're rebuilding all mechs, polishing parts, repinning connectors, upgrading assemblies, etc, that same swap can take 2 or 3x.

So it just depends.