1 - 2 horas
My wife dropped her iPhone 4S for the 2nd time in 6 months. First time, the glass only cracked in the corner. Second time round the whole display fractured. Called the local telco Starhub for a quote to replace the display assembly; worked out to some USD 300. A quick check on iFixit.com learned I could do this a lot cheaper myself. And so I did; ordered the parts @ ifixit.com and followed the guide. Easy peasy.
The first step in the repair involved removing the Apple-specific 5-lobe screws. The iFixit Pro Tech Toolkit comes with the tool for the job. Problem was that the quality of the tool was less than expected: it didn't fit so had to file it down quite a bit. After that I managed to remove the 5-lobe-screws and the rest of the steps where straight forward. Took me about 2 hours to complete the whole guide and the phone looks and works like brand new now
Start with a clean desk, allow for plenty of time and make sure you write down which part goes with which step.