10 foot. It is not an actual whip but an extension of the Kinetic Blade Infusion that lasts longer and has the reach property.
Amd just like Kinetic Blade, the appearance doesn't matter. It doesn't gain any special properties of the weapon you emulate. It doesn't gain the critical threat range, damage dice, damage type, or other properties of the weapon you emulate.
But it's also only 10 ft if you're a small/medium creature. Larger and smaller creatures have adjusted natural reach, so those sizes have different length Kinetic Whips.