1 - 2 hours
Bought my iPhone 4 when it came out way back when. Over time the lock button began losing its 'click' and eventually stopped working all together.
Ordered this part, followed the tutorial here on ifixit, and had it up and running like new I'm under 30mins :D
Have some way to keep the iPhone screws sorted. There's a lot of different sizes and it's sort of hard to tell them apart. I used bottle caps and just labeled them based on which step of the tutorial they came from.