Arbitrary number generator....7.
There's really no magic number, I don't think. It depends on, to name a few things, how many things there were to flip to begin with, where the flip is, how close together each curveball is, in what context, subplot or main plot, how quickly you can deal with it, what kind of things you're changeing, why the change was made, and how much it makes sense.
I mean, I can't very well say, "you can throw one major curveball every 10,000 words," because while someone probably has, it doesn't really mean anything. I think you have to make sure the plot makes sense more than anything else. At the point things stop making sense, you have a problem.
I would be careful not to do too many reversals, but it's kinda a subjective thing. The length alone doesn't give me personally much to be able to help you with.