How often do captchas appear?
Those rules are not true:
- one edit every 30 seconds
- one answer every 60 seconds
- one question every 60 seconds
This was first answer in few hours. Also no questions where asked. During answer submit SO asked to add more characters to answer body. After adding characters, captcha was displayed.
UPDATE: Imagine situation, when you need to add one more character. Then you have to wait 30 seconds before posting answer? Not good.
BTW good usability advise - if there are less than minimum allowed characters in answer body, then just make submit button disabled, or show error text on client side before submitting answer to server.