5 - 30 minuten
The original factory HDD in my MacBook Pro 15" Unibody (Mid 2010) was failing, but still bootable. Wanted to get a new HDD in before it fully died and before my newborn came (because I knew it'd be harder to ever get the repair done after she was born).
It went so well. The delivery of the replacement Hard Drive was a day after the guaranteed delivery date, which kind of stunk because I had paid extra for the two-day shipping. Anyway, instead of coming the day before the baby was born, it came after she was here, so I had to keep an eye on her while I did the repair, which was challenging. Luckily, the instructions were so clear that I was able to get the hardware part done in probably about 30 minutes. Then, thanks to a plan that I had worked out in advance with some iFixit folks, the formatting and Time Machine restore of my stuff was done in just a few hours more.
Don't be afraid to do a repair like this, even if you think it's going to be too much work, or too risky. You can do it. And if you have questions, just ask iFixit. They're great.