Most of the time I've used Kroger's own "All Purpose Ammonia". I can't see any data on concentration. I think once on a jug from ome other store I may have seen 5%. I've gotten very strong darkening. I've occasionally used household ammonia from other chain stores. Mostly the darkening depends on the individual tree, and often a specific part of a plank. Stress wood will darken more than regular heartwood. I just leave it in the tote until the darkening is what I want, or doesn't seem to be getting any darker. Two days is generally enough, and putting finish over it darkens it further. It will NOT yield the same degree of darkening every time. Sapwood (white oak) will not darken much. I use household ammonia since, like you, there is NO way I would use any of the more concentrated versions. As I said above, I use it a lot with white oak, but very rarely with any other wood.
Thanks to all who contributed to the discussion, particularly the "fuming" idea, which was new to me.
What a great place, Sawmill Creek woodturning! You come for the Ooohs and Aaahs, and stay for the new ideas, complete with all the "how-to's" and the "do's-and-don't's", all the stuff you need to know.
Robert