My experience with stormwatch mode (having had it activate 3-4 times since my Jan 6th 2020 install) is that it seems to be set to run for the period of the Weather service alert. For example, in our case, it was activated each time for "high wind warnings" that could down power lines. These alerts showed up in my weather app (dark sky) as alerts from the national weather service for the area I live in.
They also had a time frame, like "from 11am on 02\19\2020 until 4pm on 2\21\2020" and stormwatch mode was in effect the entire time of that alert, whether we had actual high winds in the area or not.
In the above example (dates made up), it would have turned off at 4pm on 2\21\2020... even if there was no high winds starting on 2\20\2020 in my specific location.