Personally, over swell heights over 1m and period over 10 seconds and I take notice, but there are so many variables like small changes in swell direction that can affect if a particular anchorage is tenable or not.

Adding to the confusion, not all sources seem to agree: Windguru is showing me a 16 second 2m swell THIS MORNING, while PredictWind is predicting 0.6m and 9s :-/