I did find a reference in Dogen to "Four Hours Zazen", shorthand for the total sitting time each day in many monasteries of the Sung and the Practice Dogen had observed in China (page 143 here) ...
http://books.google.co.jp/books?id=D...zen%22&f=false
However, again, what is good for Dogen and his boys in his day is one thing. What is right in this moment for you is one thing.
Dogen constantly reminds us that True Sitting is beyond long or short time. In Zanmai-o-Zanmai he taught ...
The Buddha Śākyamuni, sitting with legs crossed under the bodhi tree, passed fifty small kalpas, passed sixty kalpas, passed countless kalpas. Sitting with legs crossed for twenty-one days, sitting cross-legged for one time — this is turning the wheel of the wondrous dharma; this is the buddha’s proselytizing of a lifetime. There is nothing lacking. This is the yellow roll and vermillion roller [holding all the Sutras]. The buddha seeing the buddha is this time. This is precisely the time when beings attain buddhahood.
Upon coming from the west, the First Ancestor, the worthy Bodhidharma, passed nine autumns in seated meditation with legs crossed facing a wall at Shaolin monastery at Shaoshi Peak. Thereafter, his head and eyes have filled the world of the land of Cīnasthāna till now. The vital artery of the First Ancestor is just sitting with legs crossed. Prior to the First Ancestor’s coming from the west, beings in the eastern lands had not known sitting with legs crossed; after the ancestral master came from the west, they knew it. Therefore, for one life or ten thousand lives, grasping the tail and taking the head [i.e., from head to tail], without leaving the “grove” [right where one is], just sitting with legs crossed day and night, without other business — this is the king of samādhis samādhi.
Gassho, J