Just like any other medicine, there are many variables that affect what dose is effective for the condition you are treating. Some of these variables include individual genetics and metabolism, fed state, drug tolerance, and route of administration.
CBD is found most commonly in oil / tincture form and can be taken sublingually or ingested. The sublingual route has a faster onset and may have a duration of 2-4 hours at therapeutic levels. If added to food or beverage / swallowed / ingested the onset is slower (1-2 hours) but the duration of action is longer (6-8 hours).
CBD is often taken for chronic pain, migraine headaches, and prevention of seizures. As CBD products typically have a low THC content, you may not feel any effects from administration other than alleviation of your symptoms. This is beneficial for many patients as they find that they can still be highly functional in terms of cognition while undergoing treatment. While not psychoactive, CBD does have some mild sedative / relaxing effects and may be helpful in patients with anxiety and/or sleep disturbances.
Most patients find that one to two drops or sprays are sufficient for their needs, taken anywhere from every 1- 8 hours.