According to the ps3trophies.org trophy guide for MGS4 the writers personal estimation is 8/10.
It takes at least 8 playthroughs on different difficulties and approx 80-100 hours to get all emblems.
http://www.ps3trophies.org/forum/metal-gear-solid-4-guns-patriots/167253-metal-gear-solid-4-trophy-guide-roadmap.html