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Asphesteros |
![Red Dragon](http://cdn.paizo.com/image/avatar/17-red-dragon-FINAL.jpg)
Rules on falling say A character cannot cast a spell while falling, unless the fall is greater than 500 feet (or the spell is an immediate action, such as feather fall). So by inference 500 feet is about a standard action's worth of time, and safe to say if the fall is less than that, it's instantanious (like an immediate action). But since a round is 6 seconds and rate of fall is same as in real life, google & math can figure it out exactly.