While we cannot give a definitive answer to a person's needs, we can give you a general guideline to start with! Remember that everyone is different. You may find that after your "vaping technique" is developed, your nicotine needs change. There are two different techniques, the "mouth-to-lung" inhalation technique, in which vaper is pulled into the mouth and then inhaled (similar to a cigarette draw), and the "direct-to-lung" inhale, where the vapor is pulled directly into the lungs (like taking a breath of air) and immediately exhaled. Mouth-to-lung devices generally do not produce as much vapor as a direct-to-lung device would, and since nicotine is cumulative, the more vapor you have, the more nicotine you ingest with each draw.
A general guideline is:
For Mouth-to-Lung Vapers:
If you find yourself getting nauseous or head-achy, the nicotine strength is too high. If you still crave cigarettes, you might want to consider getting a higher nicotine strength.
Remember that nicotine levels in the body are cumulative- meaning that the more you vape, the more nicotine you get.