The amount of THC in a vape pen hit can vary greatly depending on the product used.
Type of product: THC vape cartridges can come in a wide range of concentrations. Some might contain as little as 50-60% THC, while others can contain over 90%. The higher the THC concentration in the vape liquid, the more THC will be in each hit.
Size of the hit: The amount of THC in a single hit can also vary depending on how deeply and for how long you inhale. A larger, deeper hit will typically deliver more THC than a smaller, shallower one.
The vaporizer setting: Some vaporizers let you adjust the temperature, which can affect how much THC is vaporized and inhaled in each hit. Higher temperatures will typically vaporize more THC.
As a rough estimate, a single hit from a vape pen might typically deliver between 1-5 mg of THC, depending on the factors above.