From my (new) reading of Velocity Control, what with the car and projectile examples, it's a spell without a Duration. i.e. a one-time speed bost, as if the target were pushed by force to increase their speed.
After the spell is cast, physics works normally with air resistance and friction and all that jazz and the target decelerates at a certain pace, depending on circumstances (including, if desired, deceleration from the Levitation or Flight spell)
A month's duration of Velocity Control would make it redo the speed boost in ritual casting time intervals, a horrible, horrible curse.
All things considered, go with:
Primary Factor: Duration
Suggested Rote Skills: Athletics, Science, Survival
The subject flies, fluid and free. She instinctively
creates the telekinetic force as she maneuvers in midair, granting
her incredible speed and maneuverability. The subject gains
an air Speed equal to the mage’s Gnosis + the spell’s Potency + its Speed.
While airborne she can make a Dexterity + Athletics roll to avoid
obstacles, gains her normal Defense against attacks, and can fly
without exhausting herself as she would by running.
And use Velocity Control to temporarily multiply speed for a sort of overland-flight kind of thing.
Or just add the +Speed thing to Levitation's text as a homerule thing. They are example spells, anyone with the Gnosis and Forces has the mystic knowledge to know this can be done and how to do it. If you feel it too good with the Speed boost, make it another Reach option.