10 - 15 minutes
My iPhone 5 power button had gotten to the point where it barely worked. The battery was not so great either, so I figured I'd swap that out while I was in there.
The instructions that iFixit provides are clear and accurate. If you take your time, keep the screws organized and don't put undue force on anything, it will go fine. The only thing that irked me was that I sprung for the 54 bit driver kit and the 000 ended up stripping one of first screws I removed. I switched to the cheap phillips head included with the battery kit and it worked perfectly for the remainder of the disassembly.
When putting the logic board back in after swapping the power button cable, remember that the power button cable has a connector that runs to the underside of the logic board. If you don't seat this prior to inserting the logic board, the board will wobble and you'll risk warping it if you start putting screws back in.