I've always read this trope as an episode or event where there's a plot twist that changes status quo. Thus, it doesn't really work well in a series that already doesn't have a status quo, unless the twist changes the way things have been changing previously. In other words, a twist far greater than any other twist in a series.
To have one every other episode doesn't really speak for the trope, and feels more like easily overexcited tropers at large. Personally I'd limit it to one or two occasions per series (unless maybe it's a Long Runner
Edit: To be more specific, it's not about a specific numerical limit; it's about patterns. If there's a pattern of huge plot twists, you're just not going to be as surprised by them, and they won't change your perception of the series.
edited 1st Jul '12 12:01:58 PM by Feather7603