I would expand on that and say within 15 mins through 2 hours. I have probably had close to 20 books added over the years to different passports and although the time has increased and has taken as much as 2 hours, I have never heard or seen it take 4 hours. SBM, that's a new one, I have never heard of missions abroad wanting to mail them back, and for that matter I cannot recall any thread anywhere on FT where this has been stated (and there have been many threads on the subject).

I should also note, as it seems that the knowledge all has to be restated everytime, that some missions do require scheduled meeting times for ACS, however that they are often available online and possible even same day.