Depends entirely on the information that's being messaged...
If you're communicating how many parts the line has run, you can send the message once per second or so. If you're communicating an encoder position for something motion specific, you might need to send the message continuously (or more likely, through produced/consumed tags). If you're communicating a FIFO list to keep track of users who have logged into a protected area on the HMI, you could message it once per minute.
There's flexibility here for a reason, and it's your job as the programmer to determine how much of a delay between the messages being sent is the longest amount of delay possible that will still accomplish your goal.