1 - 2 horas
I ignored the multiple red lights and beeps that my PS3 would suddenly turn off with for a few weeks / months. For some reason, it didn't cross my mind that one of my best friends might be dying a terrible heat death. Sadly, one day (in the middle of Uncharted 2), he gave out on me and turned all....dead. The PS3 shut off as it had done many times before, but this time I was unable to simply turn it back on. Upon trying, I received the dreaded Yellow Light of Death (YLOD). Of course, I needed to rescue my buddy (also, my data was at stake!), so I scoured the Internet for a solution....that brought me to iFixit.com.
I took a LONG time to repair the system, since I obsessively went through cleaning EVERYTHING. I think I took about 5 hours one night opening it up, clearing out dust and (cat?) hair and IPA-ing the crap out of it.
After a few days of of gorgeous PS3-guts art in my apartment, I spent another 5 or so hours taking my time and putting the thing back together. The guide was great, but I did have to mix in some info from the 'Gilksy' repair videos on YouTube, especially for reassembly.
I was very meticulous about thermal paste application and also placement and orientation of the thermal pads. It has been ~3 days since the repair and I have had no indication that anything is amiss. The system works like a dream.....and the fan no longer sounds like a cat in a blender.
Best of luck, noble soul! Please search around for some very good (detailed) reassembly instructions and, I would recommend, buy the iFixit repair kit. Especially since you get a heat gun, the price is good and the addition of everything you need makes it a convenient and fair deal.
As for what to do once your PS3 is back on, there is no guarantee that it will last, so *back up your data.....all of it.....ASAP*. There is a lot of info online on this subject, but the gist of it is that you'll need a FAT32-formatted storage device that can hold all of your PS3 data (you'll need to use third-party formatting software if the device is over 32GB and/or you are running Windows 7).
BEST OF LUCK!