2 - 4 horas
I to wanted boost performance by adding an SSD (though I went for a Hybrid in the end since the SSDs are still pretty pricey).
The only problem I had was getting the antenna plate back in. I could only get one side fully slotted in. I took it apart & reassembled multiple times but it just wouldn't fit. In the end I simply left one screw out. Annoying but what can you do.
Get some post-it's, write a step number from the ifixit guide on each and stick them to (or under if your working on a table without any sort of tray) each part as you remove them. Since the guide doesn't show you how to reassemble this makes it easy to simply look through the guide in reverse and reach each part as you come to the step number it's labelled with.