DrLang wrote:6-7 hours would sound about right allowing adequate time to eat and sleep. Though the mantra for Kokuzo Bosatsu is quite a bit longer than Amida Nyorai's name, I think you would just need to push harder and could still get enough sleep especially if you had someone else preparing your food. I am still curious if there is a prescribed daily schedule or if you're supposed to just go for it.
About sixteen or more hours a day are required for the set period, usually 50 days, but also 100. Akasagarbha's mantra is recited one million times over that period. There is a formal sadhana followed that is practiced twice a day, the mantra recitation with mudras and visualization is imbedded in that. It is not just "going for it" by reciting the mantra on the bus or during whatever you are doing until you reach one million. There are a number of subsidiary practices to be followed daily. Food is not taken after noon, and as a rule you eat once a day. The food is typically a paste or gruel made of buckwheat flour, you prepare it yourself. You have no contact with the world during the period. Everything is done in isolation and only in certain remote mountain temples, you speak to no one and see no one. Food and sleep are kept to minimal survival levels.
You cannot attempt this potentially dangerous practice without a teacher, and you must have finished shido kegyo. This practice is secret. More than this information is beyond what may be discussed in public, already I have said too much and will say nothing further on this subject.
The mentioned book takes its information from Yamasaki's book Shingon, which is unfortunately mistranslated or mistakenly edited in too many places to rely on except for the most casual reading.