The weather changing intervall may be up to 96 hours and there may be only minor changes. Sometimes only the windspeed changes from 4 to 7 m/s and than back again.
Most of the time the weather system works quite good, but like in real life there is allways something to grumble. You'll see when you're in storm and fog conditions for several days.
If you save your game often, the intervall may restart everytime you do so and your impression confirms itself.
Take the weather as it is.
