Easier than I thought!
15 - 30 minutos
My computer sounded like a blender. The fan went out just because it was about 4 years old.
The whole process took about 30 minutes from start to finish. It was much easier than expected, and the instructions were vey helpful and easy to follow.
When I took the top casing off, I totally forgot that the instructions had just told me to do it gently. I ended up nearly ripping off the keyboard/mouse cable in my impatience over not being able to loosen the plastic fasteners in the front. Just try not to get so impatient, and remember this important instruction, and you'll be completely fine.