CBD is “biphasic” – meaning that in lower doses it can make one alert, and in higher doses can make one sleepy. Everyone’s ECS is different, so it’s best to start slowly. It takes about two weeks to fully feel the effects of CBD – after 10 days to two weeks, you can titrate up or down slowly until you find your “sweet spot.”
Typically, one goes by weight and severity of issue to find a “jumping off point.” If one’s dose is 20mg a day, take 1/2 (10mg) in the AM and 1/2 (10mg) about an hour before bed.
I would start with the lowest dosage possible, with the brand I use CBD Biocare it starts at 500mg, I am currently using the 1000mg which I take 3 times a day for my lower back pain. Not sure what you are taking it for but for basics check this article out.