throttle messages to no more than one every 5,000 milliseconds, see http://jsfiddle.net/firebase/VBmA5/

bolt-rules-throttle.json

type SharedWithInviteRequest {
........
  createdDate: RateLimiting
}

// in order to write a message, I must first make an entry in users/${uid}/rate_limiting/{feature} is Number
// additionally, that message must be within 500ms of now, which means I can't
// just re-use the same one over and over, thus, we've effectively required messages
// to be 5 seconds apart
type RateLimiting extends Number {
  validate() { this >= now - 500 && this === root['users'][auth.uid]['rate_limiting']['xyz'] }
}


path /users/{$urlUserId} {
  // the new value must be at least 5000 milliseconds after the last (no more than one message every five seconds)
  // the new value must be before now (it will be since `now` is when it reaches the server unless I try to cheat)
  /rate_limiting/{$throttling} is Number {
    read() { isOwner($urlUserId) }
    write() { isOwner($urlUserId)  }
    validate() { $throttling.test(/^(xyz)$/) && this === now && (prior(this) == null || this > prior(this)+5000) }
  }
}