When an alert is first created in Helios (either by Helios itself or from the NWS), an effective time attribute is included that designates when the alert became active. We can query for extended duration alerts using the effective_max query option:

https://api.helios.earth/v1/alerts?event=poor+visibility&effective_max=now-60m

In this case, our search results will include Poor Visibility alerts that are at least 60 minutes old. Any shorter duration alerts will be filtered from our result set.