Research regarding CBD is still relatively new for dogs. However, based on the evidence found in humans, studies suggest that one dose of CBD taken orally will have a 1.2 life of 1-2 days.
A half-life is the amount of time it takes for 1/2 of the dose to be eliminated by the body. However, this is just an estimate. As everyone's bodies are different depending on the metabolism and how fast or slow their unique enzymes take to break the compounds down.
Another thing to note regarding the amount of time it stays within their body is remembering that cannabinoids are fat-loving. They build up in the body's fatty tissue over time if you use them daily or regularly.