When the idle is set correctly, an engine would theoretically idle all day long, without loading up.

When you come in for a landing, on a long, steep final, you could be idling as long as 30 seconds. If when opening up after a missed landing, the engine just "barfs" and dies, you've got trouble.

If the engine would not open up immediately after 30 seconds of idling, it is not adjusted properly.

Sincerely,