Here's what I've figured, so far, out that works for me. 1) Pregame, pregame, pregame. Decide together ahead of time whether you'll call out of your primaries. 2) If there is nothing to referee in your primary, and you've agreed to call in your secondary, you can best help by watching the off ball action. 3) If you are going to call something on ball near your partner, you had better be certain that your partner was unable to see what you saw. And even then, you probably want to pass on it unless it's a foul that "has to be gotten."
To address your direct question, not very often
