Im guessing you mean how far as in not numerical, if so then it would extend as far as the Hakurei Shrine but the most you know of that is that it's on the eastern edge and since there isn't a map of Gensokyo you can't really tell how far. Anyway official works, you say?
Perfect Memento probably shows how far it extends if you count this "On the eastern edge of Gensokyo lies its only shrine. This shrine exists on the border between the outside world and Gensokyo"
Bohemian Archive mentions this "Deep within the mountains, seperated from the human village, Gensokyo's only occupied shrine can be found situated on the border between Gensokyo and the outside world.However, while this shrine does have a connection to Gensokyo, it's not a particularly close one.It doesn't have a close connection with the outside world either. This shrine's location is on the very border."
Ghostly Field Club with this "My partner took out a picture and started talking as usual."But never mind that, Merry, why don't we go see the entrance at the Hakurei Shrine?""
So the barrier I guess, would extend to the (somewhat) outside world (if you look at it that way) but there isn't really a location for the "outside world" Hakurei Shrine.