def next_in(key, count, duration)
      @key = key
      @duration = duration
      return 0 if current_count(key) < count
      [@duration - elapsed, 0].max
    end
def next_in(key, count, duration)
      @key = key
      @duration = duration
      return 0 if current_count(key) < count
      [@duration - elapsed, 0].max
    end
Gets the ETA for the next window start only if the limit has been breached.
 Returns 0 if the limit has not been breached.
 @param [String] key
 @param [Integer] count
 @param [Integer] duration