45 minutes - 1 hour
My Xbox360, after 3 years of solid use, starting having video problems. The video would be all messed up, and then it stopped showing a picture all together. The audio continued to work though. Lots of posts online seem to agree that this was an early symptom of an impending RROD!
I decided to try the heat gun reflow and also install the RROD fix kit. Everything went great, although I really took my time and it probably was about 2.5 hours total, most of which was disassembly, removal of old thermal paste, and reassembly.
I haven't tried using the console for an extended gaming session yet, but so far so good!
Everything worked great, the guides were pretty thorough.