So my observation is that any amount I've used has been ok, but then I've never have all that much relative to the scale I'm using it at.
Apparently though, it takes a lot of ash to raise pH. This extension bulletin suggests 1 point increase at 10-11 tons per acre
(see figure 3)
That's a lot more than I would have thought. I'd guess I'm using the equivalent of 1/2 a ton per acre at most. Theoretically that should only affect pH sensitive crops like spinach. But on the other hand, if your soil pH is already acidic you may want to use quite a bit.