15 - 30 minutes
For a while there had been a noise coming from the right side of my MBP. It got to the point of at times sounding like a blender.
I have replace the hard drive before so I was familiar with the tear down process of the MPB. With the instructions found on iFixit the process when very smoothly and I was up and running quietly in less than 30 minutes.
Print out and follow the instructions, make sure you have the correct tools. It is really pretty easy and don't be intimated by the insides, take your time.