1 - 4 horas
There wasn't any problem really, I just wanted better performance and a longer life out of my computer. Because of the dropping prices on SSD's I decided to go for it!
The repair went as well as I'd expect given that this was the first time I've taken apart an iMac and considering I didn't have a great work space to do it in. The removal of the Logic Board was a big pain, as well as reinstalling it. There was a few times that I was afraid I had damaged something and it wasn't until I got everything back together and turned my iMac on that I knew I didn't mess anything up!
Just to reiterate what was already said in the guide, make sure you have a great work space. Something that has a hard surface like a table, counter top, etc. I completed my repair off of a couch and my bed. All in all, find the correct iFixit guide for your project and follow each step carefully and you'll be fine.