My thought has been like around 25% of the initial range.
So if you start casting as a max of 40 yards, you can cast and not fail until 50 yards.
Again…just athought…never measured / timed it.
I do this on purpose at times. Wait to cast when the mob is pathing AWAY at max range. Sometimes they pat too far and fail, but most of the time it goes off with extra distance.