I wish the mist lasted longer than an hour. Looking at my weather seed, all the mist is at 7AM and no other times.
The mist starts at 7 AM, but it sometimes lingers into 8 AM. The weather seed tool won't tell you when it'll linger, but it's not as thick. However, by 9 it's definitely gone.
I have also seen a light mist an hour before a rain storm before in the middle of the day. It was definitely foggy, though nowhere what it was at 7 AM. Again, the weather tool doesn't seem to indicate when this happens, though like contrails it may not be pre-determined.
I'd personally love to see the really heavy mist last well into the day. It's such a cool feature. I can understand why it only lasts an hour, but for aesthetic reasons, it really should happen more often. Especially at night. Where I live, fog can linger until noon and came arrive as early as 7 PM on some days.