I think you are asking how long you system will be affected after you smoke. If this is the case, then a typical ‘high’ from a vape pen will last anywhere from 1.5-4 hours depending on how much you ingest. Yes, it does depend on the person and the interaction with their system as well and it also depends on your tolerance. It is also dependent on how much THC is within the vape pen. There are pens that go anywhere from 5%-95% for instance. That is a huge difference.
If you are asking how long a vape pen oil cartridge lasts, this too depends on how often you use it. For me, a vape pen cartridge lasts a very long time as I tend to take 2 puffs and then I put it away. Other people may take 10-15. So, this is variable as well.
I am not a doctor, this represents my opinion and is taken from my experience only. I hope this helps!