As per the spell description...
The altered weather affects a 2 mile radius (3 if the caster is a druid). If you are outside of the radius, you are not affected, though I'm certain you could see it. Note that it is centered on the caster. The caster can move, which would take the effect along with the caster, so keep that in mind.
The duration is 4d12 hours (double for druids), so the weather remains at whatever the caster sets it at (within the limitations of the season as given in the table in the spell description) until either the duration expires, or the caster changes their mind and sets a new weather, whichever comes first. Note that the change is gradual, so attentive characters may be able to find or make shelter before the storm gets to the point of having to make saving throws.
The Wind Effects listed affect valid targets every round so long as the wind remains at that strength, or they are adjudicated to have total cover against wind effects (which is quite a trick, unless you can find or make an underground/underwater/extradimensional shelter in a hurry). I believe that this general assumption is based on the fact that when the wind can do damage, it does said damage every round; therefore the rest of the effects are per round as well.
Note that making the saving throw negates all wind effects listed in the table for that round.
Having personally ridden a bicycle during a "Severe Windstorm" strength storm in real life, I can personally attest that fighting the wind really is a round by round activity. I had to lean about 40 degrees into the wind on my bike just to stay "upright and balanced". The wind and rain was so strong it caused all my bike spokes to howl loudly due to the sheer vibration of the wind passing through them. At times I would simply slide sideways slightly, which is quite an unnerving experience.
Not that personal experience and/or real life physics counts for anything with game rules, but I thought it a fun share, at the least.